The first transmission of any kind that I worked on was a Dual Range Hydra-Matic in 1958. (I was 12 years old) My father was the first man in Nashville Tennessee to rebuild automatic transmissions, beginning in the late 1940's, and by the late 50's he was teaching transmission repair courses for General Motors, after hours in the family shop. I sat in on the classes, worked alongside the mechanics who were training, and passed the end-of-course test. GM sent me a certificate stating that I was a "factory-trained Hydra-Matic technician"!
No matter what vehicle the transmissions happened to be installed in, the internal workings were practically identical. Olds, Pontiac, Cadillac, GMC and Chevy trucks, Nash, and a few other car lines all used that transmission- - - -even Rolls-Royce and Lincoln Continental used the H-M at various times. Thousands of WW II 6X6 trucks and the Stuart light tank also used an earlier version of the H-M before the Dual Range shift pattern came out in the mid-1950's. The Stuart used two Cadillac flathead V8 engines and two transmissions.
The original Hydramatic came out in 1938 in Oldsmobiles and some Buicks- - - -with a manual clutch. It would shift automatically, but the clutch had to be disengaged at a stop. By 1940, a fluid coupling was added to let the engine idle in gear.