I've noticed that my Plasmat-type 210, 240, 305 are longer (dimension H1 in the catalog, front rim to back rim) in shutter than in their barrels. Varies from .5mm to several mm depending on the shutter and lens, older Compur, Synchro-Compur, shutters. All the shutters have the correct threads and correct 20mm thickness. Curiously, the old Tessars and Ektars that came in the Compurs maintain their length when put into the G-Claron barrels. Also my one Dagor-formula G-Claron (210mm) maintains the same length when moved into a shutter. Something about the threads of the modern G-Clarons? The threads of the G-Claron barrels start a few mm below the rim of the bushing, and the lens cell threads appear to have a widening tha still fits into the unthreaded part of the barrel bushing but stops them from screwing further in once it encounters the shutter's threads. The cells stop screwing into the shutters when the widening hits the threads; but keep on screwing into their barrels until the cell's shoulder lands on the rim. The increase in length is shortest (.5mm) in a Polaroid Copal, but it too has threads starting several mm below the rim of the bushing.

Could this be by design: an increase in spacing perhaps might give better corrections at longer distances, where lenses in shutters would be used? (Pure speculation on my part). If not, would a small change in focus have any serious practical effect? The focal length will change a tiny bit, but perhaps at f22 and below no bad things happen. I have been happily using my 305 G-Claron (3mm longer in a shiny 1969 Graphic-branded Synchro-Compur from a 127 Ektar) oblivious to this issue, and it is stunningly sharp.