thanks Fisherman,
I found it interesting in the book regarding The Divine Plan of the Ages published by C.T.Russell in 1886. Ephesians 3:11 reading 'according to a plan of the ages' in the Emphatic Diaglott, and in the New World Translation it is eternal purpose like in the book title.
In the book God's Kingdom of a Thousand Years Has Approached (9173) pages 206/207 it explains how Russell calculated the 6000 years of man's existence on earth to end in 1872 and by allowing two years between the creation sinning they arrived at 1874 for the end of six thousand years
'This resulted in their calculating six thousand years of sin as ending in 1874 C.E in the autumn, the seventh millennium began, for the instigator of sin, Satan the Devil , to be bound and cast into the bottomless pit and for Christ to start reigning for the foretold thousand year period.'
On page 209 it says in 1943 they did away with 100 years due to finding something in Judges that altered the chronology and 'This moved forward the end of six thousand years of man's existence into the decade of the 1970's.' This also did away with 1874 as the date of the return of Christ.
So this 6000 years and the devil being cast in the bottomless pit seems to go back a long way.