No mystery at all. It's definitely been worked on over the course of it's service as about 90% of them have. The metal finish is still original and not FTR. It looks and reads like a very nice rifle. Canadianowned rifles aren't rare but not that common to find either.
I just looked and unfortunately don't have a Mk.3 catch for your rear sight. I do have lots of spare springs for them. I'm scrambling like everyone else now to find spare Mk.1 and Mk.3 rear sights for shop restorations. The only complete sights i have available are Mk.2 of all different mfg, British, Canadian and US.