Check out Vol. 1 from SITS by CTR, it contains the Chart of the Ages and the chapter Plan Of The Ages, 1886.
For those interested in the 'Generation' issue' this will help in seeing the underpinnings of the belief and the evolution of the idea and the Two Classes. Russell's version was different than Rutherford's version but both believed that the central purpose of 'this Age' is for the election and final selection of the Bride Class. Those familiar with the old terms recognize things like 'refining', 'chiseling', 'perfecting' of the saints.
Check- The 'Gospel Age' and the 'Harvest' shown on the chart and explained by Russell. A reworking of Calvin's 'Election of the Elect' doctrine.
The 1919 solution to the great disappointment of the Bible Students has now become the modern Watchtower's solution, a open ended Last Days.
Those clever guys, they tried to sneek it in without revealing its source or telling the Witnesses.