Regular visitors to this site would have seen the occasional round photo inside a square black frame. This is no digital trickery like where you see an old Polaroid frame around a modern digital image. These are genuine fisheye images taken on film and here is the culprit.
Now my oldest lens, a great campaigner and veteran of many briney missions. It's in semi-retirement until somebody comes up with a cheap way of removing the fungal growth inside one of the many pieces of glass that enable it to focus light from 180 degrees onto a piece of film.