In 1993 when the Proclaimers book came out this was buried away in it. I do believe that information on the internet forced them to reveal their past. I have never found any other reference to 1878 before this in WTS publications.
*** jv chap. 28 p. 632 Testing and Sifting From Within ***Based on the premise that events of the first century might find parallels in related events later, they also concluded that if Jesus’ baptism and anointing in the autumn of 29 C.E. paralleled the beginning of an invisible presence in 1874, then his riding into Jerusalem as King in the spring of 33 C.E. would point to the spring of 1878 as the time when he would assume his power as heavenly King. They also thought they would be given their heavenly reward at that time. When that did not occur, they concluded that since Jesus’ anointed followers were to share with him in the Kingdom, the resurrection to spirit life of those already sleeping in death began then.
*** jv chap. 28 p. 632 Testing and Sifting From Within ***
That 1878 was a year of significance seemed to be fortified by reference to Jeremiah 16:18 (‘Jacob’s double,’ KJ) along with calculations indicating that 1,845 years had apparently elapsed from Jacob’s death down till 33 C.E., when natural Israel was cast off, and that the double, or duplicate, of this would extend from 33 C.E. down to 1878.
*** jv chap. 28 p. 633 Testing and Sifting From Within ***
See “TheTruthShallMakeYouFree,” chapter XI; “TheKingdomIsatHand,” pages 171-5; also TheGoldenAge, March 27, 1935, pages 391, 412. In the light of these corrected tables of Bible chronology, it could be seen that previous use of the dates 1873 and 1878, as well as related dates derived from these on the basis of parallels with first-century events, were based on misunderstandings.