It is a BSA COMMERCIAL rifle, BUILT as a Target rifle for long-range competitions.
The rifles had to be built to military SPECS for the matches they were used in, but they were built slowly and very carefully.
A few (and this is one) then went to small, private shops (A.G.Parker, Parker & Hale, George Gibbs, Holland & Holland, Alex Martin in Glasgow) where they were worked-over yet again to become the finest Service Rifle competition pieces in the world.
Under the rules of the old Service Rifle competitions, the rifles had to start as identical to an Army rifle, 100% INSIDE the military specs: no heavy barrels, bobbed bayonet lugs or anything like that. The rifle COULD be modified for the shoots, but any modification made HAD to be able to be removed and the rifle returned to battlefield condition with NO TOOL OTHER THAN A SCREWDRIVER. This meant that fancy precision sights were okay, but they had to come off fast.
When the competition came, you showed up at the firing line and your rifle was examined to make sure that it complied with the regs. Then you were ISSUED your ammo for that target. It could be anything that the Army had in storage - any make, any year, any Lot number, any Mark - but it had been stored properly and met military specs and that was ALL you knew about it. For a 600-yard shoot, you needed 15 rounds, so you were issued 17: the extra 2 rounds were supposed to be enough to let you sight-in your rifle for that ammo, at that range, and figure out how the rifle should be held for that ammo, at that range, in those wind and temperature conditions.
It was the most difficult competition in the world and it is one which has passed out of fashion in this day of cheap optics, heavy barrels and super-accurate special ammo.
But that was the competition for which your rifle was built...... and it was built to WIN.