If GM followed previous conventions on casting codes, that E 2 5 might be a date code signifying
April May 2, 1955
(or maybe 1965). That plate where the E 2 5 appears was on a small plate screwed to the sand casting form each day, leaving the impression on the sand mold when the case was cast.
But according to the 1955.1 and 1955.2 Vehicle Information Kits, 2-speed transmissions were identified with Plant code T (for Toledo Plant) and then a number for the month and a number for the day of the month. So I don't think that would make your transmission a 1955 year build. Unfortunately the 1965 Vehicle Information Kit doesn't provide codes for engines and transmissions like the earlier ones do. There may be a serial number stamped on the right rear of the case (likely on a machined surface) that might provide a clue to when it was built.
[on edit] fixed the month (can't count letters correctly
) Also, the SM420 was replaced by the SM465 in 1968, but the casting number changed after 1963 like Dan found.
