Given that it's in the same spot each time, then it's not a problem in the back, nor is it a strap/cap or it'd be black.
I'm voting for pinhole in the shutter. Try to think back to when you took the photos, were the ones with less leak (like the Smellican), shot sooner after the previous (in which case, less time for a pinhole leak to invade), and the ones with more leak (like the first) had longer time sitting behind the shutter for a pinhole to get that bad?

Or, it's a leak when the shutter is opened, coming from somewhere in the space twixt shutter and lens. Were the worse leaks longer exposure times, and the lesser leaks shorter times?