Gee, those must have been very, very, very tiny machine parts no thicker that the planchet, itself. Right? You're letting your imagination get the best of you. Chris
It was caused by... C'mon. A set screw somehow getting into a planchet? You can't seriously believe that's what you see on the reverse of the coin. This is about the 3rd or 4th damaged coin you've posted and then gave some "off the wall" explanation after experienced members gave you an accurate answer. Go to sites like variety vista, Error-ref, doubleddie.com and spend time studying real errors. You may want to check out Fred Weinberg's site (or other credible error/variety dealers) and study the errors he has for sale. If you're searching the web, I would be very careful about what you view. Lots of people out there claiming to have errors that don't know anything about the minting process