The Powermatic transmission is actually an Allison 6-speed automatic with a retarder pedal located where the clutch pedal would normally be (looks like a gas pedal), this allows the transmission to be use for down hill speed reduction saving wear on the brakes.
There were changes/improvements made over the years from it's introduction in 1956, for example in 1958 they added a second Drive range and changed the look of the shift tower.
The different applications are identified by the color of the Allison Data Plate (like red, green, yellow) that were riveted on the side of the trans case.
Mike B
