It is my understanding that quite some time ago a Cdn collector obtained a lot of about 400 of these stripped .22" receivers. As far as I know, the ones seen floating around are from this batch.
I, too, remember the LB Mk. II Sten main casings being sold. Might even have an old surplus store catalogue listing Sten parts. They had the casings, butts, mechanical parts, but no bolts, barrels or magazine housings. Then the CF cut their remaining Stens, and there was a flood of demilled guns, in varying stages of demilitarization. That solved the problem of magazine housing availability. The registered LB guns Lee Enfield mentionned were probably assembled during this period. At the time, assembling and registering one of these was not a problem.
Never saw a Bren receiver.
It was possible to purchase C1/C2 rifles, C1 smgs directly from LB. Wouldn't have believed it until I saw brand new in the packaging guns, and the paperwork. Chap just thought to ask, and found out they could be purchased.