As Captain Laidlerhas yet to reply to your inquery, I thought I'd bring my VERY limited insight to the discussion.
My first No.2 Mk.I** was (is) a short bbl. version. Cut back, yes, but the ramp front sight IS broad arrow marked, and the whole thing was phosphated and Suncorite'd, so I'm guessing it had some sort of official sanction. Definitely not a "Bubba".