My understanding (from reading a popular book) is that the Sten Mk5(S) barrel was fixed within the silencer module and the head spacing flange on the barrel was mostly machined away to clear the barrel fixing pin of the Mk5 tube, and a head spacing washer, split to clear the pin was used to adjust head-space.
Nice work by the way...I'm wondering when I can visit texas....this year looks like I'll have actual time available to leave, the issue is whether I'll have a reason to come back.
I find it fascinating that the Sten Mk5 is for all intents back in production (in parts form) in the US
Indianapolis Ordnance - STEN MK5
Indianapolis Ordnance - STEN MK5