It fits the inner side of the ring where there is no thread; not the threaded (58 mm) outer side. This makes it possible to rotate freely into the correct position. Thereafter it has to be fixed with glue. If you look carefully at the uploaded photo, you can see the 58 mm thread of the reversing ring.
Ah, that makes sense. Looks so clean that I didn't suspect that it was just glued together.
Going to see if I can get the parts to make one myself. I have a K3 on hand.