Got Photos?       

David,

The numbers you need to be looking for are stamped, not cast, into the transmission. These numbers will the yield the year and date of the transmissions assembly. The letters and numbers are usually very lightly stamped and hard to find but a wire brush and some 100 grit sandpaper will reveal them. Look at the side cover of the transmission and see where the machined surface of the transmission case extends beyond the side cover plate. That would be forward of the side plate on the drivers side. Some times the same numbers are stamped in a second place which is found on the forward face of the transmission. This second location cannot be seen unless the transmission is out of the car. Check the attached photos. On this transmission I decode the "H" being 1950, the "A" being January, the "31" being the last day of the month, and the rest of the numbers being the serial number.

From what I can deduce the code for the years runs as follows

B= 1942-1945

D= 1946

E-H= 1947-1950

J=1951

K=1952

Following 1952 the years were indicated directly 

53=1953

54=1954 

etc.

The months were sequenced alphabetically with the exception that they skipped the letter "I" going from "H" to "J" ...

@lou-macmillan

This came attached to a 1962 235 s/n F0405J

I tried to attach this with the pictures, but I guess I did it wrong, so I am attaching pics now

.  This came attached to a 1956 235 engine with solid lifters.

@bob-remm Thanks for your information, I have stamped numbers on one that I found: G0 43670  

I am attaching it just in case I am reading it wrong.

@david-jewett The long tail shaft is a three speed form a 55 or later car with an open drive line. The short transmission is from a pick-up truck with a closed drive shaft used up until 1954.

    I looked thru a 1957 parts book but could not find any casting numbers that matched.  The torque tube trans came out of an Advanced Design 1948 - 54 3-speed light commercial.   If it has bushings on the counter shaft it came from a 1948 - 52.  If it has bearings in counter shaft it's 1953 - 54.   (1953 fleet vehicles used a 216 engine with narrow belt and a bushing transmission.)    

    GM replacement parts will fit but are not necessarily original.   If the engine in your 1940 school bus pushed a rod thru the block of it's 216 engine in 1954, GM would sell you a 261 engine with your original serial stamped on it.  This why the casting number is more important than the serial number to determine originality.            

@lou-macmillan thank you for the information!

@david-jewett   the long tail shaft trans. is from a truck  ,not car.   This is detremoned by the 2 lower mounting ears being treaded so the 2 lower bolts are inside the bell housing .     a car uses all 4 mounting bolts outside the bell housing .