Peter: seen this low axis pin on many No4s?

[Surpmil]

Here's a photo of two I had; 1931 on the right side and 1935 on the left. (Not a very good photo as it is a jpg of a 35mm print.)

One can see how the 'shoulder' that extends from the rear sight axis hole 'lug' along the bolt way 'makes sense' in the original design, but as modified from 1935, there is a piece of the shoulder 'left hanging'.

Notice that the area milled out is 'in the white'.