You write:-
Can anyone tell me (authoratively) whether the 16C74A is one of the
parts that can't be erased once code protected (UV parts).

Yes its one of them !

However the method of hiding the code protect fuse involves putting an
metalisation layer over it dependant on the thickness of layer determines the
length of time to erase some product will erase if left in the eraser for
several hours. I have personal experance of this left them in the pot for a
weekend and on monday they worked fine. Dont give up just yet .

Cheers Steve.....