While we are on the history of 4-speeds topic....
The new "Muncie" book by Paul C. of 5speeds.com has a lot of great information on when certain design changes were incorporated, casting numbers for components, etc.
http://www.5speeds.com/bookviewer.html
I bought a copy, and I think it's a good resource if you are building / ID'ing Muncie transmissions.
Anyway... the one thing I learned from the book is that GM actually designed the T10 and had it produced by Borg-Warner. They later licensed it to Borg-Warner, and made money off of everyone Borg-Warner sold in the '60s. The Muncie was an evolution of the original T-10 design.
I knew that the transmissions were closely related, but I always thought GM "reverse engineered" the T10 to develop the Muncie. I didn't know that they developed the T10 in the first place (working with Borg-Warner Engineers, no doubt).