I once bought a Rodenstock 75 f4.5 in a Copal 0 that was 'funny' unscrewing the front assembly resulted in it coming undone after 1/4 turn and falling away from the shutter. The casing was such that the assembly pressed on the front of the shutter and instead of screwing home, pressed on the dial face and as it was tightened instead of coming to a halt, prevented teh speed dial turning. Therefore the assembly was entirely different. I can only assume that as the front did not properly screw into a copal 0 (correct shutter) that it was DB and therefore had an entirely different external geometry. Needless to say I returned it!!!! this is the only case of what I thought to be a DB lens/copal situation that I have encountered and there is no way that lens could have been used in a regular shutter.