Wheel Horse transmissions fall into two main categories:
The geared transmissions either being designed and manufactured
by Wheel Horse or Peerless.
Hydrostatic transmissions manufactured by either Sundstrand or
Available as a 3 speed+reverse on early models.
Later marketed as a 6 speed or even 8 speed with
the addition of a high/low transfer box.
Typically 3 speed+reverse apart from some 6 speed
models fitted to B series tractors. These were also the transmission
used on vertical shart engine models.
Early hydros used on charger, GT14 were hydrogear/piston.
Later placed by piston-piston types - approx 1973 onwards.
more info here
Used on many hydro models from 1976 onwards