[DWThomas]

I got full coverage on 4x5 (inch) film with a pinhole about 63 mm (approx 2.5 inches) from the film, so I'd say there is something in the way. If the pinhole plate is truly 3/4 inch behind the original lens mount, that could be it. It is surprising what odds and ends can get in the way. My first SQ-A body cap pinhole adapter had serious vignetting and it was an issue of what seemed like large clearance holes being far enough before or behind the pinhole plate to impinge on the intended cone of rays. In that case, I did an accurate scale drawing -- and "oops!" Used the scaled layout to more accurately determine the necessary dimensions and the second round was fine.