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Looking for GA drawings for 1923 XM-1 boxcar
The 1923 proposal shows up on p.120 of the 1925 Car Builder's Cyclopedia (ignore the Enterprise drop doors in the floor), helpfully available here:
I would assume the 1925 revision was in the 1928 CBC, and the 1931 CBC had the 1929 revision.
RPC #18 is also a good resource here.