ST400. That's what Buick called em and that's good enough for me. If it was a Powerglide inside, wouldn't matter- ST400 is what Buck called it and that's what I was saying in my post when I was "corrected". It seems that the shifting of the ST400 was different than a TH400 as well.
The 64 also is lacking in controlling down shifts, it shift 3 times but only has a low and drive. If you want to hold it in each gear you start off in L, wind it up and shift to D, that puts it in second gear, now pull it back into L and that holds it in second, wind it up again and shift to D and you are in third. Here is the part I don't like, say you are driving in the mountains, the turns are tighter and slower, you drop in down to L holding it in second gear, then you have to slow down to about 25 mph to make a tighter turn and bam! it goes into low, then you faster, put it into D to put it back in second and and back to low to hold it there.WHEW! I drove a 64 Riviera everyday for 13 years so I know.