About the time I was born, early 1949, my father purchased a 1947 513S. When I returned from service he gave me the rifle. When I was 40 the rifle was stolen from my home during a move. Last year, I found it for sale at gun show. As it was my main shooter as a boy, I knew it well enough to recognize the dings I had made and the serial. When I explained the situation to the seller, He showed me his C&R bound book entry and offered to sell it to me at his cost. I was delighted to accept. Meanwhile, some previous owner had tapped the left side of the receiver for a diopter sight mount. Consequently I would like to find such a mount. When measuring the taps does one measure from outside edge to outside edge, inside edge to inside edge, midpoint to midpoint? How do I determine the size of the taps and the threads? Obviously, I am not a machinist or a smith.