I suppose this is technically a medium-format question, but it seems the answer to it can only lie in this forum. I have a Rolleiflex SL66 and have been adding various boards to my lens arsenal. One board that came into my hands recently is a bit of a mystery. It was listed as being L39 (I was planning on mounting enlarging lenses on it), but is in fact slightly larger. My digital calipers put the diameter of the inside ot the threads at 39.6mm (1.56in). I've tried a Copal No.1 and it drops in without contacting the threads. That is the only metric-dimensioned shutter I can think of that comes close. I've been through listings of every lens mount known to man and nothing matches. Given that someone once bothered to get a blank board and have it machined to these dimensions I can only presume they must have had something good and suitable to mount on it. Any ideas? Anyone know of a listing of US shutter thread dimensions?