I don't think they all were. How many wires are on the plug going into the trans? That is an easy identifier. are you looking to order parts or just identify what you have? Buick kept the ST name for a few years in the beginning, the other makes called it the TH400 right off.
ST is Buick's designation "Super Turbine" and the "ST" has nothing to do with the Variable Pitch (Switch Pitch) feature. The ST models, both 300 and 400 did have those that were Switch Pitch feature. (But other ST300 and ST400 models did not have Switch Pitch feature) As stated, it was Buick's "name" for the Turbo-Hydramatic models. BT 68 it is a 1968 "big car" with 430 engine. (I have a 430 and BT tranny out of a 968 4D Wildkitty setting in my garage, in the corner, keeping the mice from turning my cars into Condos)
Trans tag is that of a 68 BT Non Variable pitch trans. That dual dimple pan, without the "Turbo Hydramatic" script was used from 68 to 72. It's getting to be rare, and is one that resto guys in the know search for, since it is correct for all GS and GSX cars of that era. If you replace it, I am sure someone on the board here would want it, don't throw it out, it looks to be unrusted and undamaged. All 400's from 65 to 67 were Variable Pitch transmissions. Not sure when the ST designation stopped, and the TH started. "Switch Pitch" is a term Kenne-Bell coined in the late 60's amd early 70's for the 65-7 transmissions, or any later TH unit that was equipped with the VP pump and input shaft. JW
Technically, that is a Super Turbine 400, but as mentioned, it was Buick's name for the 400. A 1968 400 would have been fixed pitch as the Switch Pitch was only from 65-67. As far as when the 400 transmission's name changed, I say in 1969. My evidence comes from the Buick Chassis Manuals. In 1968, The manual says ST300 and ST400, In 1969, it is Turbo Hydra-Matic 350 and 400, but still Super Turbine 300,
Creeping towards GM ubiquitous corporate identity. Great answer Larry! Chassis manuals can be a source of knowledge and inspiration. Tis where I found out the 46000 series had a 4-speed. A grand spiral since then for me.
Don't plan on throwing it out. It's going be rebuilt and used in a Skylark I'm building. This is my 1st journey out of the Chevy zone so I'm trying to learn. I do appreciate all the information!