+1 on Magellann's post above, adding......
....and it looks pretty nice!
The Mark 1 rear sight is a nice sight but it is slow to adjust. Because of the amount of milling necessary to make it, and that wonderfully-fine micrometer-screw adjustment, it was the bottleneck in production. It was suprceded by the Mark 2 sight: the little 300 - 600 flip sight. The Mark 3 sight as on this rifle, was preferred, even though the Catch stuck out too far. This sight has the modified Catch on the Aperture and likely could be called a Mark 3/4 sight. It is fast and quite solid and MUCH better than the little Mark 2. AFTER the War, many sights were replaced on FTR with new-made Mark 1 sights. They all interchange: the Mark 1, 2, 3, 3/4, 4, C3, C2 and any others which might be lurking in the woodwork.
NICE rifle.