This forum has been such a gold mine of knowledge that I hoped someone here would be able to help with another gear question. I bought the Pentax-M 100mm 2.8 on FleaBay and as I focus and head toward infinity, I noticed it would stop and jam up. Then I shook it an noticed the rattling sound and that there was a ring between the front element of the lens and the mount that was loose. I can get it back in place but if I adjust focus again, it shifts and I'm not able to change the focus until I realign the metal ring internally. Any idea what that ring is and to fix it? Do I just return the lens? Uploaded a video here to hopefully help illustrate the issue: