Sparky: You're quite right about the 1939 map. I looked up the proposals and they did indeed plan for two extensions beyond Euclid Avenue. They also had two proposals for the Rockaway Line. One was to connect to the Queens Boulevard Line and the other was to connect to the Fulton Street El.
The IND Fulton Street Line reached Rockaway Avenue in 1936 and this became a temporary terminal. Some work beyond this point must have been done before the war interveined because the extension to Broadway-East New York was made in 1946 and the one to Euclid Avenue was in 1948.
I guess that I will have to stop by the transit museum store and pick up that map.
Best Wishes, Larry, RedbirdR33