Wow, I never knew these even existed.
This is what Wikipedia has to say:
The Roto Hydramatic was used in all full-sized Oldsmobile models including the Dynamic 88, Super 88, Ninety-Eight and Starfire from 1961-1964 as well as the compact Oldsmobile F-85 from 1961-1963. Pontiac used the Roto Hydramatic from 1961-1964 on its shorter-wheelbase full-sized cars including the Catalina, Ventura and Grand Prix, but continued with the older four-speed Super Hydramatic design in the longer-wheelbase Star Chief and Bonneville models. It is believed Pontiac was made to take Roto in the short wheelbase models ( Catalina, Ventura, Grand Prix) by the Corporation. The reason behind that was Oldsmobile'* sales volume alone was not profitable enough to use Roto all by itself, therefore Pontiac was used because at that time Pontiac was having tremendous sales and in fact from 1961 to the end of the decade was number three in sales behind Chevy and Ford.