This item was in a box of enlarger lenses, lens boards, enlarger part and miscellaneous other darkroom equipment. I thought it was a retaining ring for mounting an enlarger lens since it's a pretty standard 39mm opening. The threads are VERY far apart, so I don't know what male thread it would attach to. The nearest thing I can find online that might match this is a mounting ring for light sockets. I found a similar item described as "Lamp Shade Phenolic Screw Collar Rings For Medium Base E26 Socket". I am listing it as darkroom equipment since every other item in the box was identifiable darkroom gear.