BZZZZT. Incorrect. And the board goes back. There was never a Hydra-Matic with two speeds. If the transmission is original in this car, it's the two-speed Jetaway, offered starting in 1964 on the A-body Oldsmobiles as well as the low-end 88s (Jetstar, Delmont) for several years in the mid- and late-'60s.
Jaunty - Would that have basically been the same transmission as the 'Roto-Hydramatic' used in the 389 '64 Pontiac Ventura I owned? I had read somewhere that the Grand Prix and Bonneville models got a true 'Hydramatic' behind the 389 and larger engines, but the lower lines only got the 'Roto-Hydramatic'. And it sure seemed as if it had more than 2 forward speeds to it. Seemed more like 3 or 4......
No, I do not believe so. The Roto-Hydramatic was a three-speed automatic. In 1964, all four of GM's non-Cadillac divisions had a two-speed automatic for their lower-end cars. Olds called its the "Jetaway," Chevy's was the "Powerglide," Buick called it the "Super Turbine 300," and Pontiac, boringly enough, just called theirs "Two-Speed Automatic." As I understand it, the three non-Chevy transmissions were similar and might share parts (but I don't know for sure), while the Chevy Powerglide was a completely different beast and doesn't share parts with any of the other three. That's why I cringe every time someone with a mid-'60s Cutlass says that their car has a "Powerglide." To Chevy-centric people, any GM two-speed automatic from that era is a Powerglide. But us normal people know that that's not true.
And if you are old enough to remember riding in a Buick of that era you would know that it did not have any Chevy powerglide - Buick trannies were way smoother and even had a different sound.
The Buick and Olds versions of the ST-300 and TH-400 from 64 to 67 used the variable pitch torque converters while Pontiac used a fixed stall speed converter.