And the next day ...
I've been thinking (smell the smoke!) the fact that there appears to be light struck area on the film rebate pretty much means it's a seal problem. The frame mask is between the film and the body shutter, and even the dark slide and the back to body joint. The rebate is more open to exposure between the spools and the insert rollers.
There -- how's that for profound pronouncement!
They are really neat and capable cameras, but the back light seals appear to be a weak spot. And of course virtually any backs around are at least a dozen years old -- and maybe thirty!