I bow to your superior knowledge of the Service but can only say that I do not know, but it does. The only thing I can think of is that my service has a barrel that comes further back in its fitting than is normal. I do not know how that can happen but will have a look to see ... the barell detaches easily and nicely when the push button is pushed, no play when engaged.
Seems to be a puzzle; but if the breech seal was any thicker it sure would stick out into the breech opening a long way.
I did get some leather from a 'bag maker' once but thought it very expensive just for a small offcut.
Cheers, Phil