Ah...I believe that I misunderstood what was being talked about previously. You're talking about the thread size that goes between the lens and the mount. I have a few lenses branded by Soligor and Vivitar that use a setup like this so they can build one lens to fit several different mounts. I never knew what they were called. Mine all have the set screws to properly orient the lens too. Thanks for the clarification. Unfortunately, I can't tell you what the thread size is. I've attached a photo if that's any help to anyone.