Mk1... no such thing as a No1 mk1

Seriously, great score! The rear sight has never been modified for MkVII ball, the hump in the ramp is the give away. It would not be correct for a rifle modded to Mk1***. I think the hole in the top of the sight cap is the staking screw which was fitted to stop the sight being fine adjusted. Look out for centre punch marks in the bottom of the cap to lock the windage screw. Not sure when the sights were alterred, but I don't think it should have been done without changing the ramp angle for MkVII... (does the number under the sight leaf match the barrel serial?) Have to dig the books out to be sure.
Another member here showed me a Mk1 barrel a number of years ago but I think it did not have a serial on it. I wonder if it's the same tube?