Didn't Keyser run two ST's one was factory built and the other was an S converted to ST spec? I believe he also used it as a Camera car in Speed Merchants.
The way I understand it is the factory S/T's did start out as S's... And yes, Keyser did have 2, one with the 2.5 race engine (plus a spare 2.5 engine), and the second, the camera car, with a 'stock' 2.4S engine...

Cheers,
cm