Are we talking about brinell or Rockwell Hardness testing? These dont look like that. and in actuality you put a hardened steel ball against the metal and press against that ball with a known weight and measure the distance across the indentation (diameter) and compare that distance to a series of known values and that will give you an indication of the hardness. It would not be nessessarry to do four tests so near each other in such a small area because one would usually give sufficient indication of hardness. Also from a cosmetic perspective one would be enough and often, even in military weapons the test is done in an area where other inspectror stamps are stamped. usually in an area where they canot be worn off or damaged during normal use so that in the future if it needs to be verified that the test was done during some recall as was the case with the notorious early o3's

Would somebody have centerpunched the reciever to drill holes for some sort of scope mount.