I spent a while myself trying to figure out what that was - hahah. My pinhole camera is made out of half inch thick plywood and the hole that was cut out in the wood on the front for the pinhole to go behind was too small at first. The hole being too small resulted in light bouncing off of the wood and framing the image. After I finally realized this I made the hole larger and you can see with the bricks and door image that I almost completely fixed the problem. After that photo, I made it even larger but haven't had a chance to try it out yet.
Ok. In essence your original hole formed a mechanical vignette hood?