Rattigan350 : However it was clear that the 144,000 was filled by [1935]. The first resurrection started in 1918 after the heavens were cleansed.
That is what was more or less believed between 1966 and 2007. In the May 1, 2007 Watchtower there was a Question from Readers where it discussed previous understandings and concluded "it appears that we cannot set a specific date for when the calling of Christians to the heavenly hope ends."
Further, the January 1, 2007 Watchtower said that the idea the first resurrection started in 1918 was an interesting possibility but is not directly confirmed in the Bible. In fact, all that can be said is that if Jesus was enthroned in 1914 then the first resurrection probably started after that because there is no mention of them when war was waged in heaven against Satan and his demons. Personally, I do not find that evidence compelling.
Listener : You say it is fact that one of the 24 elders revealed the identity of the great crowd to John. How could they reveal it to him if none of them were even in heaven at the time?
As Revelation is about events "in the Lord's day" (Revelation 1:10), the 24 elders can represent the future 144,000 even though none of them were in heaven at the time the vision was given. The book Revelation - Its Grand Climax at Hand, 2006, p.125 para. 17 says that the identification of the great crowd by the elder suggests the resurrected ones of the 24-elders group may be involved in communicating divine truths today.