The Hydra-Matic transmissions in this thread are 4 forward speed transmissions. The 1954 GMC Hydra-Matic shown at the link above had an optional 2-speed reduction unit, resulting in 8 forward speeds.
The GMC Twin Hydra-Matic (available in 1955 and maybe later) has 7 forward gears/speeds, because it is two 4-speed hydra-matics sort of mounted on top of each other and sharing one output shaft (it should have been called the Siamese-Twin Hydra-Matic).
The 21 speed GMC Twin Hydra-Matic is a 7 speed Twin Hydra-Matic with a 3-speed reduction unit, providing 21 forward speeds (a 2-speed reduction unit was also available). I have a GMC Shop Manual for these transmissions.
It would be very interesting to see one, and even more interesting drive a truck with one.
