Thanks, all, for the replies. I think I'll proceed with trying to create a better slot in the head and then perhaps richard's idea of adding some form of grit or compound to provide better grip for the screwdriver.
Tom, you have been fortunate, indeed. All of my film backs were acquired as used items, and almost every one proved, on its first use by me, to need new seals. (and totally regardless of external appearance, naturally). So now whenever I get another back I immediately change the seals. The parts are pretty cheap, thankfully!
Originally Posted by Tom Nutter