Barbour already knew of the 1873 date. It was common in the literature, which Barbour well knew. And I think what he says in Evidence suggests he already had Elliott's Horae in mind. All of those who had already pointed to 1873 or 1874 or both dates were Anglican or Church of Scotland. The sole exception is a German Baptist clergyman. In another place he suggests that Miller pointed to 1873 as a possibility. I think that's fable, but it at least shows that the date was in his consciousness, and that it wasn't original with him.
B gave me permission to post this from Nelson Barbour: The Millennium's Forgotten Prophet. This is from the developing revisions to that book:
It would be surprising indeed if Barbour were unfamiliar with Elliott’s Horae. He calls it a “standard work,” which it was; it was very influential among American expositors until at least 1900. He found in Elliott a satisfying Bible chronology which relied exclusively on the Bible’s internal evidence and ignored secular history and opinion and from which he could support his 1873 date. “I was looking to 1873,” he wrote, “and when I saw this chronology supported the argument of the 1260 and 1335 days of Dan. 12, naturally examined it with interest and ... have never seen a scriptural reason to abandon it.”
The chronology was developed by Edward Elliott and Christopher Bowen, the Anglican Vicar of Southwark. Bowen was a prophetic student in his own right and published a pamphlet entitled Things to Come Practically Enquired Into in 1847. He contributed a chronological table to Elliot’s Horae that was a restatement and revision of Henry F. Clinton’s chronology, incorporating revisions to it made by Elliot. Bowen’s table is distinctive enough to call it a new chronology and contemporaries differentiated between the two. Daniel T. Taylor prints the two chronologies along with others in a table. Clinton and Bowen’s charts differ by ten years. Because Bowen crafted the table, he is credited with the chronology. In truth, credit belongs at least equally and maybe primarily to Elliott.
Barbour read John Aquila Brown’s The Even-Tide; or Last Triumph of the Blessed and Only Potentate, a two-volume work published in 1823, either then or earlier. Brown was of interest to Millerites because he ended the prophetic 2300 days in 1844, and his book was well enough known in America that the anonymous author and publisher of Watchman of the Night and Millennial Morning and Peters in his Theocratic Kingdom could refer to it without further explanation. Matthew Habershon, an author recommended by J. A. Seiss, referred to him. Isaac Wellcome mentions Even-Tide and connects Barbour to it, though he says, “Barbour... terminated his time argument at the same point, but by a different mode of reckoning, in some respects.” One would conclude that Barbour read Brown’s Even-Tide, even without Isaac Wellcome’s statement.
Key elements of Barbour’s prophetic scheme are found in Even-Tide, which seems to be their ultimate source. Brown would have interested Barbour because he ended the 1290 days in 1873. Barbour wouldn’t have read far into Brown to discover this. It is on the frontis chart in volume one and on the title page. Though Barbour doesn’t mention Brown or Even-Tide, he admits that others had pointed to 1873. He quotes Miller as saying after the 1843 disappointment that he could see “no light this side of 1873.” He saw Miller’s words as a seal to his own conclusions; yet, he wrote “others have said as much or more” about 1873.
I could not locate the German language pamphlet, but a secondary source suggests that John Boger (1774-1852), a German Brethren clergyman, promoted a similar chronology. His pamphlet’s exact title is lost. In English it translates as The Coming of Christ so it may be Das Kommen Christi. Unaware that Russell received the date from others, one of the authors of a history of West Virginia wrote: “John Boger was author of the pamphlet printed in German on ‘the second coming of Christ,’ a treatise that fell into the hands of ‘Pastor’ Russell and formed the foundation for the Millennial Dawnists Church.”
John Fry in Observations on the Unfulfilled Prophecies pointed to 1873. Fry ended the 1260 days in 1872-73, writing that “the arrival of the years 1844, 1872, and 1889 must be expected with feelings of the deepest interest by all who are looking for ‘this great day of the Lord.’” W. Snell Chauncy also pointed to 1873 in his 1839 publication Dissertations on Unaccomplished Prophecy. In 1835 Thomas Brown suggested that the 1335 prophetic days might end in 1873, and he felt the way was opening up for “the full triumph of the Gospel kingdom and the final restoration and conversion of Israel.” Matthew Habershon counted the 1290 days from 583 to 1873-74 C.E. (A.D.). At least one advocate of 1873 was mentioned in The Literalist, printed by Orrin Rogers in Philadelphia between 1840 and 1842. Closer to Barbour’s time, the anonymous British writer “S. A.” suggested in his Apocalyptic History that at least one prophetic period might end in 1873. Though the basis for fixing on 1873 varied, there were a number who believed it a prophetically significant date.
 N. H. Barbour: Chronology, The Herald of the Morning, August 1875, page 38.
 Burke’s A Genealogical and Heraldic History of the Landed Gentry of Ireland (London, 1899, page 3) contains the following brief biographical note: “Rev. Christopher Bowen, M.A., of Hollymount, and Heatherwood, Isle of Wright, formerly rector of St. Thomas, Winchester, b. 16 Oct. 1801; m. 17 Jan. 1834, Katherine Emily, dau. of Sir Richard Steele, 3rd Bart, of Hampstead, d. 1890.” Dictionary of National Biography, Supplement, Volume I, New York, 1901, page 238, says that Bowen “was successively curate of Woolaston, near Chepatow, and of Bath Abbey church, rector of Southwark, and rector of St. Thomas’s, Winchester.”
A brief profile of Christopher Bowen is found in his son Edward’s biography: “The Rev. Christopher Bowen was the eldest representative of an Irish family holding property in county Mayo, and was distinguished by many of the best characteristics of the Evangelical school of those days – devotion, spirituality, a hearty dislike of ecclesiasticism, profound earnestness, unquestionable sincerity. He was also very gentle and affectionate, full of sympathy with the trials of others, full of compassion for their failures or mistakes. Sir Henry Cunningham, in his memoir of Lord Bowen, has referred to him as ‘an excellent reader, whose children enjoyed no greater treat than to lie on the hearthrug and listen to his rendering of one of Shakespeare’s plays.’ He was, too, one whose able mind remained fresh and active with advancing years. ... It may be added, in view of the striking poetical gifts of his two elder sons, that he was a somewhat copious writer of verses, both humorous and sentimental.... He died on the Riviera in 1890.” - Bowen, W. E.: Edward Bowen: A Memoir, Longmans, Green, and Company, London, 1902, page 8.
 C. Bowen: Things to Come Practically Enquired Into, Binns & Goodwin, Bath, and J. Nisbet & Co., London,  1849 edition. Other works by Bowen exist. This is his only work on prophetic subjects of which I am aware. Bowen’s chronology doesn’t appear in the booklet. The chronology seems to have been drawn up specifically for Elliott.
 D. T. Taylor: The Reign of Christ on Earth: Or The Voice of the Church in All Ages Concerning the Coming and Kingdom of the Redeemer, H. L. Hastings, Boston, 1883 revised edition, pages 538-540.
 E. B. Elliott: Horae Apocalypticae, Second Edition, London, 1846, page 254-259.
 John Aquila Brown was a silversmith and a Factor or agent for one of the Sheffield silverplaters. He appeared as a witness at the Old Bailey on April 6, 1826, in the case of Elizabeth Wood Lloyd. He declared, “I live in Bouverie Street, and I am a silversmith.” (Sessions Paper: The Right Honourable William Venables, Mayor. Fourth Session, Held at Justice Hall, in the Old Bailey, On Thursday, the 6th of April, 1826, and Following Days, London, 1826, pages 316-217.) He died in March 1849. His birth date is unknown. His wife’s name was Ann E., maiden name unknown. They had two sons, Daniel born March 1, 1814, and David born January 1, 1816. Both sons were enrolled in The Merchant Taylors’ School.
Brown is occasionally described as a Church of England clergyman. This is incorrect. Brown belonged to the Philo-Judaean Society. He sponsored or seconded a resolution at one of their meetings and is included in a list of “movers and seconders.” All listed are given the title “Rev.” except Brown who is listed as “Mr. John Aquila Brown.” – The Missionary Register For M DCCC XXVIII Containing the Principal Transactions of Institutions for Propagating the Gospel, London, 1828, page 229.
In 1808 Brown was one of the principals at an organizational meeting for The Church Missionary Society for Africa and the East. He lived in Pall Mall then. - The Early History of the Church Missionary Society for Africa and the East to the end of A.D. 1814, London, 1896, page 193.
 Volume 1, pages 26-32, 107, 118-119 cited by Froom, Prophetic Faith, Volume 4, page 266-267. Though Brown’s Even-Tide was printed in the UK, there are many more copies in American libraries than in British libraries. One gets the impression that Brown’s primary audience was in the United States.
 G. N. H. Peters: The Theocratic Kingdom of Our Lord Jesus Christ, Funk and Wagnalls, New York, 1884, Volume 3, page 99.
 Matthew Habershon: A Dissertation on the Prophetic Scriptures Chiefly Those of a Chronological Character: Shewing Their Aspect on the Present Times, and on the Destinies of the Jewish Nation, James Nisbet and Co, 1834, page 197.
 Wellcome, Isaac: History of the Second Advent Message and Mission, Doctrine and People, Yarmouth, Maine, 1874, page 477.
 N. H. Barbour: Evidences for the Coming of the Lord, page 34. Despite extensive research, I cannot find the source for this quotation. It seems an uncharacteristic statement for Miller to have made. Isaac Wellcome, questioned the quotation. – History of the Second Advent Message, page 370.
 Written by an anonymous committee: History of West Virginia, Old and New, American Historical Society, 1923, Volume Three, page 489.
 John Fry: Observations on the Unfulfilled Prophecies of Scripture: Which are yet to Have Their Accomplishment Before the Coming of the Lord in Glory or at the Establishment of His Everlasting Kingdom, Printed for James Duncan and T. Combe, London, 1835, page 380. This book is in the British Library.
 Published by James Nisbet & Co.; J. Johnstone, 1838, page 387. This book is in the British Library.
 Thomas Brown: A Key to the Prophetical Books of the Old Testament, Published by the Author, London, 1858, page 103.
 Matthew Habershon: A Dissertation on the Prophetic Scriptures Chiefly Those of a Chronological Character: Shewing Their Aspect on the Present Times, and on the Destinies of the Jewish Nation, James Nisbet and Co, 1834, page 452.
 The Literalist: Elements of Prophetical Interpretation, etc., Orin Rogers, 1840, page 333.
 S. A.: Apocalyptic History, S. W. Partridge and Company, Second Edition, London, 1871, page 21.
 Peters mentions a Balfour who looked to 1873. This seems to be a misprint for Barbour. – N. H. G. Peters: The Theocratic Kingdom of our Lord Jesus Christ, Funk and Wagnalls, New York, Volume 3, 1884, page 99.