I just bought a Schneider 90mm f/6.8 Angulon (to use with Ben's upcoming camera) and there is a little screw -- shown in the box in the picture -- that is not flush and cannot be screwed further in. It prevents the retaining ring from making a flush contact with the back of the lens, so there is no way to mount this firmly to a board without a wobble. Anyone know what this screw is and how to resolve this issue?

John