I just purchased a Ruger Alaskan 454. I noticed some markings on the back face of the cylinder. An "SM" and 3 small dots approximately the size and depth of light center punched marks.
I assumed that the center punch marks were from a rockwell hardness tester. I'm a machinist and have used these hardness testers in shops before.
I called Ruger and asked about these marks. The guy I spoke with took my number and told me he would call me back. Within 5 minutes, I got a call back. He stated that the cylinders on the 454 alaskans go through 3 separate hardness testings , and that the "SM" denoted that the gun had been proof tested.
I was curious and just wanted to know. I was happy to know that someone from Ruger responded so quickly.
Just thought I'd pass it on.