It IS a mauser action. I don't think that mark is a proofmark. I
suspect it is the mark of the person/company that built the rifle,
or possibly the logo of the manufacturer of the barrel.
It looks like a well done 30-06 sporter built on a 98 mauser action.
Could date anywhere from the early 50's on---gunsmiths are still
building them today, although other actions are more popular now.