First domestic production GMC to have a Hydra-Matic was in a 1952 PM-150-22.
In 1953, GMC had the H-M transmission as an option in 1/2, 3/4, and 1 ton trucks (100, 150, 250).
In 1954, when Chevrolet trucks first had a H-M as an option in 1/2, 3/4, and 1 ton trucks, GMC had several versions of Hydra-Matics for almost all sizes of it trucks. This included a 21 forward gear oddity (which was basically two Hydra-Matics mated side-by-side, connected to a shared output shaft).
As stated above, the transmission ID plate (on the transmission or on the engine) would be the most convenient way to ID the transmission.
