Originally posted by sandstone So I did correctly identify the mount as a Pentax PK mount; the problem must be the apparent bend in the mount. Fortunately for me, the seller immediately agreed to stand behind the lens! Now, I wonder if that bent metal piece is something that can be removed and replaced without too much effort, or if it is part of the basic framework of the lens. I have never taken a lens apart, but may have to try.
It shouldn't be too terribly hard to take apart (be careful of pulling too hard on wire ribbons that send the electrical signals). The problem is going to be finding a replacement mount. They are not standard or generic. You have to find one to specifically fit that lens (or perhaps that and similar sigma lenses).