Listener, 1935 has an interesting background as the terminus ante quem of the first resurrection.
In the November 15, 1933 Watchtower (pp. 339-345) it discussed Jesus' parable of the vineyard where all the labourers are given payment of a penny/denarius at the end of the day, regardless of how long they have spent in the vineyard. In the article it identified the start of the day as 1919, and the end of the day (12 hours later) as 1931 (12 years later). The payment of a penny/denarius was identified as the new name of Jehovah's witnesses.
So there was some scriptural justification to believe the heavenly calling had ended by 1931.
The Question from Readers in the May 1, 2007 Watchtower says
For many years after that, it was believed that the call to the heavenly Kingdom had ended in 1931 and that those called to be joint heirs with Christ in 1930 and 1931 were “the last” called. (Matthew 20:6-8) However, in 1966 an adjusted understanding of that parable was presented, and it became clear that it had nothing to do with the end of the calling of the anointed.
So that scriptural justification fell away. The Question from Readers goes on to say :
After [1935, when the Great Crowd was identified], the thrust of the disciple-making work turned to the gathering in of the great crowd. Hence, especially after 1966 it was believed that the heavenly call ceased in 1935. This seemed to be confirmed when almost all who were baptized after 1935 felt that they had the earthly hope. Thereafter, any called to the heavenly hope were believed to be replacements for anointed Christians who had proved unfaithful.
This was based on the fact that one of the 24 elders (representing the 144,000) revealed the identity of the great crowd to John (Revelation 7:13-15). Therefore by the time the great crowd was identified (in 1935) the first resurrection must have begun. But there doesn't seem to be any scriptural reason to believe that the heavenly call had ended then, only circumstance.