Quote Originally Posted by ChuckP View Post
Why not just flip the retaining ring, center it and mount it in the Copal #0 hole? I've done it that way and it seemed OK. Am I missing some problem here? Maybe a light leak.
This works fine for most applications, but can result in the shutter threads being damaged if they rub/scrape against the edge of a metal lensboard or if the lens is dropped, etc. The idea of the original mounting was to protect the threads with the shoulder of the retaining ring. If the #0 hole can be filed or sanded a bit to allow the retaining ring to be mounted this way (usually easy with most boards), then that would be a better mount.

If not, "down-and-dirty" works just fine 99% of the time.