Theoretically, the rear sight on a 1931 National Match rifle should have no notch in the top of the sight leaf. But, over the years, I have seen several 1930's NM rifles with notches in the sight leaves. One can only speculate how they got there.
The notch in the top of the sight leaf was eliminated during WWI and was never reinstated.
If the rifle has a Lyman 48 rear sight, then the military rear sight is a replacement. When a Lyman 48 sight was used, the military rear sight was removed and usually misplaced.
Hope this helps.
J.B.