I've either got or had a Sten which I serviced at least once, but I don't recall what you describe. Maybe a later version.
The Mamba with Seac handle has even a different system. The screw-in outer fitting, that looks externally like the trigger pin plugs of the other Assos, is hollow and has two small o-rings in the hollow. Perhaps at one time one of those was a backing ring instead. Then below that is something aluminum that looks like a pop rivet. The ~1/2" long shaft of that piece penetrates through the handle housing and a small bit of the end protrudes into the hollow bore inside the handle. The mushroom head fits flat against the bottom of the trigger pin well (can't recall now but I think there's an o-ring under the head), and the plug with the two o-rings sits on top of that. The two o-rings are put under slight compression as the plug is screwed in. I assumed the leakage was around the crude 1.5mm pin, but I guess I haven't confirmed that. I just found a 2mm pin that I'm not sure where it belongs, but it's about the right length and when I get a chance I'll see if it will fit through the trigger pin passages.
The Mamba with Seac handle has even a different system. The screw-in outer fitting, that looks externally like the trigger pin plugs of the other Assos, is hollow and has two small o-rings in the hollow. Perhaps at one time one of those was a backing ring instead. Then below that is something aluminum that looks like a pop rivet. The ~1/2" long shaft of that piece penetrates through the handle housing and a small bit of the end protrudes into the hollow bore inside the handle. The mushroom head fits flat against the bottom of the trigger pin well (can't recall now but I think there's an o-ring under the head), and the plug with the two o-rings sits on top of that. The two o-rings are put under slight compression as the plug is screwed in. I assumed the leakage was around the crude 1.5mm pin, but I guess I haven't confirmed that. I just found a 2mm pin that I'm not sure where it belongs, but it's about the right length and when I get a chance I'll see if it will fit through the trigger pin passages.