I could be wrong Max, but I have seen and fixed about a zillion No4's and 5's - but surely, the actual SCREW, adjustment on the Mk1 No4 sight and the Mk1 No5 sight are the same - as is the round matching cursor slide nut. From memory, I seem to recall that there WERE different types of No5 screws but only in that the thread was only partially rolled down the shaft........