Hy MFW, Dual Range Four Speed Hydra-Matic, there were other transmissions referred to as "Hydra-matic" but they were different. The GM cars stopped using this design transmission in the mid fifties, but Chevy and I presume GMC continued to use this transmission until 1962 in trucks. Rolls Royce used this transmission (with some Rolls Royce modifications) until GM started marketing the Turbo 400, hope that helps.
