These extra holes are "Common as dirt".
A Parker Hale type receiver sight for a SMLE (No1 MkIII) can be fitted to a No4 by adding a hole in the butt socket which corresponds with the lower bell safety spring on the No1 receiver.
I'll take a picture for you if you like of one which I personally removed the sight (and sold it off IIRC) from a 1950 LB.