When the IND first opened in the 1930's, the letter designations on the trains were supposed to indicate the origin (Washington Heights, Concourse or Queens) and the Manhattan trunk line (8th Ave. or 6th Ave.) on which the train travelled. Single letters denoted expresses and double letters meant locals. I believe they were as follows (correct me if I'm wrong on any of these):
A, AA - Washington Heights - 8th Ave.
B, BB - Washington Heights - 6th Ave.
C, CC - Concourse - 8th Ave.
D, DD - Concourse - 6th Ave.
E, EE - Queens - 8th Ave.
F, FF - Queens - 6th Ave.
G, GG - Bklyn-Queens Crosstown
H, HH - Court St.
My question is: Prior to the Christie St. connection, which of these designations were actually used at one time or another, and which were never used? Was there ever, say, a DD or FF?