If I follow well, by building a mag housing AND a trigger housing myself, serializing them, I might be able to register a 19 inches barrel StenMk3 from a parts kit as a semiauto rifle.
The mk3 uses a welded in position mag housing, as I'm sure you know. My intention, once I get an answer back from the CFC, is to make an entirely new component to replace the "deemed prohibited" item. I'll make the trigger mechanism so it won't take any sten parts. If it's the mk2 mag housing at fault, I'll make changes to it's design as well, possibly using a sterling style mag release catch,
The reason I'll make the components non-interchangeable with a regular sten is so that I'm not building a prohibited frame. Not that I believe a trigger housing is a frame, or that a mag housing is a frame, but if they do, I don't feel like spending $10K to challenge it.
So, my layman opinion, and I'd like you to know I'm a retired military mechanic, and not any type of a lawyer, is that if you build from scratch an exact copy of the mag housing or trigger mechanism, and they decide that one of those is the heart (registered portion) of a prohibited gun, then you have likely made a prohibited gun. On the other hand, they could still come back and tell me that the SAS3 (modified to mk2) is OK as is.
Also, if you are merely going to make a mk3 parts kit into a semi-auto open bolt gun using the origional size tube and origional bolt system, you are making a gun that lends itself to easily be made full auto in a short period of time. This, too, is likely going to cost you a hunk of change to prove otherwise.