Here's a gif of the poster (at the JoeKorNer hosted at quuxuum.org) describing the service provided during this water main break.
In 1934, the following "train desintation signal letters" were in use:
A - Washington Heights - Brooklyn - 8th Ave Express (I assume by the map, to Church Ave)
C - Bronx Concourse-8th Ave-Borough Hall, Brooklyn Express
CC - Bronx Concourse-8th Ave Local (I assume to Chambers St)
E - Queens-8th Ave-all local stops (in 1934, it only ran to Roosevelt Ave; I assume the Manhattan terminal is also Chambers St)
GG - Queens - Brooklyn Crosstown Line (in 9134, it ran only between Nassau Ave, Brooklyn and Queens Plaza, Queens.
In 1934, AA, B, BB, D, EE, F and HH didn't yet exist although they seemed to be planned for.
Kinda makes you wonder what the train names would have been if the IND second system were ever built.
--Mark