could be that the set screws fell out and the ring is held in place by rust. good luck on gettin' that one out. try several different small sizes of allen wrench in the holes to see if one fits (not sure what size the set screw is), try putting on some rubber gloves and rotating it counter-clockwise. some have put duct-tape on their fingertips for grip. may need to use some penetrating oil around the ring.

I so need a vacation.