THIS IS IMPORTANT!!!
Here's another photo, which is very hard to see because it is off very old microfilm. However, you should be able to "see" my point. The photo shows the Guide Lug area of the oprod. Number 1 shows us that it was not hardened and you can "see" the area is cut. It states the following for number 1. The unhardened number one FAILED at about 2700 rounds. Number 2, which was hardened at the Guide Lug by Induction Hardening, was ok after 12,000 rounds. This testing was done in 1944 and 1945. You guys just have to get my books.
I have a couple op rods that are cut war-time (I think a -6SA and a -9SA) that are marked "M", was that marking done to signify induction hardening?