My great uncle gave me this 12 gauge MEC 650 press about 10 years ago and this piece came in the box, my guess is a makeshift sizer since this model doesn't have anywhere for the listed sizer ring and guide tube to be installed. I've looked all over the internet and haven't been able to confirm this though. It does seem to kind of work with the press, in the deprime station it pushes it down with the primer feed and the prime station does push it off partly but gets stuck on the way out. Maybe an after market part made at the time?