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:

https://books.google.com/books/about/Car_Builders_Cyclopedia_of_American_Prac.html?id=hIsmAAAAMAAJ

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.

David Thompson