Metal lock ring (retaining ring, jam nut) with 39mm x .75 thread for mounting lenses with "39mm fine" thread, and Copal, Compur, and other #1 shutters to unthreaded enlarger or camera lens boards.
Note: This ring is not compatible with lenses that have 39mm Leica thread (39mm x 26TPI). Please see the alternate listings for Leica threaded rings.