A bit of checking using the
GM Partswiki link found in a "sticky" post towards the top of the General Truck Talk forum shows that the "casting" number 591615 represents the "main case" for all 1948-54 Commercial (truck) 3 speed transmissions.
The date casting code "H278" can't be directly identified from info I was able to search for your "1953 3100" here:
https://www.gm.com/content/dam/comp...evrolet-trucks/1953-Chevrolet-Truck.pdf.According to that document, you first need to locate the actual transmission "serial number" which is stamped in the left side machined flat side where the shifter plate mounts. You will need to clean the flat surface just above the "H278" code and look for a set of stamped letters/numbers.
Once you have those numbers, you can refer back to the '53 Truck Specification document linked above to decode what the serial number can tell you (ie; Plant and date codes).
Helpful???
