Thanks, Gentlemen. Sensible advice.

There can't be anything loose inside, it's a simple wooden box. When I shoot in vertical format with it, I pull the dark slides out from the top. In horizontal mode, I pull the dark slides out to the right.

I think for starters I'll try sealing a freshly loaded film holder to the camera body around its entire perimeter with black photo tape, and make a test exposure. If the problem goes away (and it almost has to) then I can start narrowing down the suspect area, although the upper right side of the body, as stated by some, is more likely the culprit. I'll get it sorted out, I'm sure, with a little careful testing. Will post the results here, and thanks to all.

Never been able to get excited about small/med format pinholing, but doing this with LF is kind of fun! Now I'm thinking about 8x10 contact prints one of these days.