It looks like an aftermarket replacement target barrel, although the Aussie ones are a bit heavier than the Brit. ones, yours is more a Brit. profile, not quite as sharp at the sight taper.
I would love to see a better shot of the stamps around the breech end, they would tell where it came from.
I see what you mean about the frame being flat, I think there are a couple of models with this look, maybe one of the old hands may come in on this when they get over their hissy fits.