Very interesting.
The curved Sterling mag seems to fit in the Sten. I wonder if that is by design or a forced fit? I seem to recall that Sten mags fit in Sterling and not vice versa - but don't own them to know.
I am also curious if the Canadian mags are truly 30 rounds instead of 34 and a little shorter than sterling mags. I have some Canadian 1980s SMG pouches that seemingly are too short for 34 rd Sterling mags - maybe the Canuks used a slightly shorter mag?
Sterling mags will lock into a sten if the bolt is back, they will only function if the feed trough is widened in the sten bolt to clear the double feed shoulders of the Sterling magazine.
The "feed horn" of the sten and Sterling bolts are the same size where it passes through the center of the magazine feed path.
Canadian mags are indeed shorter and reduced to 30rds, this is supposed to be because of the increased friction caused by the conventional magazine platform.