Do you mean the ring on the outside of the driver that holds it on to the speaker?
@prof Bingo! The rings outside the driver facing towards us.
Always wanted to try a pair of Thiel 3.7, not sure if the Luxman 590 would be a good match.
@webking185 I think it will be fine. However, I will look at other pairings that are rated 2 ohms stable as the 3.7 are quite a difficult load.