The Momo horn button retainer ring is the part that secures a MOMO and Momo style aftermarket horn buttons into the steering wheel hub adapter. This goes in between the hub adapter and the steering wheel, so you have a place to snap the horn button into place.

The outside holes are 2 3/4 or 70mm, center to center. The inside hole, where the button attaches, is 1 15/16 or 50mm.