I posted a link with details of most if not all of the power amps from HH(the water!). The S500D was bipolar everything after used mosfets.
Yup and I did give it a good look through but failed to find the particular amp. One of my friends from the BBC remembers them though but has no idea of their identification details. They certainly were never cased not did they have any controls at all - just a bare bones amp chassis - they came in a cubic cardboard box with loads of packing and little else.