THM400 or Turbo 400, not sure what you guys talking about?
In 1964 Cadillac used two transmissions (not counting the commercial HD version), the 4 speed Hydra-Matic and the 3-speed Turbo Hydra-Matic. I have not found any reference in the 1964 Shop Manual to THM400 or any word with 400 attached to. Also checking my 1966 Shop Manual and 400 is not present there either, only Turbo Hydra-Matic. Serviceman bulletins for 1964 does also NOT state 400. Sales brochure does not use 400. Wikipedia do claim THM400 to be installed in Cadillacs from 1964, but frankly I don't trust that to be correct.
When checking my 1972 Rolls-Royce Silver Shadow Workshop Manual it is stated for left-hand drive cars GM 400 - 3 Speed. They even had different gear boxes depending on side of the steering wheel!
So in 1964 Cadillac did not use the word THM400 or Turbo 400 or any other 400 variable for the Turbo Hydra-Matic. My guess is that 400 was invented at a later stage and was used incorrectly for our cars. Not even sure a 1964 Cadillac Turbo Hydra-Matic is fully interchangeable with a THM400.
In 1964 Buick did use a Super Turbine 400. But ours cars are not (inferior) Buicks! (I also own a Buick) ;-)
Found a very interesting site called General Motors transmissions and they use TH400 for the 3-speed 1964-'98 electric kick down, later renamed 3L80. My guess is that here they have renamed the early Turbo Hydra-Matics not knowing better.
Just to make it clear. I have briefly scanned the manuals and could have overlooked something, but think that we should use the correct words when discussing our cars. THM400 or Turbo 400 is modern and was not used in 1964 for Cadillac Turbo Hydra-Matic.
Me and my autism/ocd can now withdraw.