The Sealing happened in 1881, according to Russell, with a small amount of prophetic backing. Rutherford plagerised the entire idea and changed the date to 1935. He didn't even bother to try to come up with any scriptural justification. The justification is that if the angels explained the concept in 1935 to Rutherford, then that must be the year for the sealing. Quite an ego.
Here are the Russell quotes.
Zion’s Watch Tower 1881 January
"…the gathering of the bride into the place of safety, will occupy a parallel of seven years of time, ending in 1881"
New Creation (1899) 1915 ed. p.95
"To our understanding the general call to this joint heirship with our Redeemer as members of the New Creation of God, ceased in 1881."
The Time Is At Hand (1889) 1915 ed. p.235
"As in the type that date - three and a half years after the death of Christ - marked the end of all special favor to the Jew and the beginning of favor to the Gentiles, so we recognize A.D. 1881 as marking the close of the high calling, or invitation to the blessing peculiar to this age - to become joint-heirs with Christ and partakers of the divine nature."
Jehovah’s Witnesses—Proclaimers of God’s Kingdom (1993) p.632
"It was also reasoned that the end of God’s special favor to natural Israel down to 36 C.E. might point to 1881 as the time when the special opportunity to become part of spiritual Israel would close."
Explanation of Chart Representing the Plan of the Ages
"The General Call To The High Calling Ceased—9
Even though the general call ceased in 1881, the door to the high calling remains open, as shown by number 9. However, it is only when one who is spirit begotten has to be replaced, (on account of being overcharged with the cares of this world, or sinful living, or loss of faith), that one, desirous of doing the Father's will, is invited to enter the narrow way as a replacement. The door to the high calling will remain open until the bride has made herself ready."