It hasn't been rounded off, it looks to be caught on some sort of lip (the lip cannot be seen, meaning the lip must be coming off the rod under the main piece). It's 2 pieces. The main piece extends above and below the outer ring. The outer ring floats on the main piece, and the main piece can move around the rod, but cannot be moved up or down. The outer ring is about the diameter of a quarter (this piece has 4 notches), and the main piece is not as big around, and a little greater than half an inch from top to bottom.
I'm going to try and get a pic for you, but I'm not too confident I can get it done. I appreciate your help, I hope I haven't confused you with the description above.