The attached photo shows the seam I mentioned in yesterday's post. Tonight I also noticed 2 things that have made me re-consider a set screw. 1) There appears to be a second (broken off) bolt(?) on the right side which is outlined in light blue. 2) Judging by the top lip, the thickness of the metal looks too thin to hold a set screw (or a bolt) securely. I agree that cleaning it would help. So would some additional photos from different angles. Plus any context (i.e. where found, what was found nearby, etc.).