Mystery Metal Stamp ? Old Timers, Welders? See Stamp Image

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I inherited some of my grandfathers welding tools about 15 years ago.He was a welder, shipbuilder, submarine welder at Mare Island, California during WWII to early 1950's.  He was also a welder for the largest Hard-Rock mining operation in California, The Empire Mine in Nevada County, Cal..He was a master welder and he used to make some his own welding gasses.I found a metal stamp in the lot that has had me puzzled ever since.  It is the type to mark metal, such as gas cylinders.The stamp is about 1/2" square, intricately made and hardened.  It shows a large "R" with a smaller "N""2" in the top half enclosed portion of the "R" and a "S15" in the lower half between the two legs of the "R".A few machinists friends have suggested that it may be a 'Welder ID' stamp for my grandfather to stamp his work at beginning and end of shift.I have contacted NBBI, ASME.  It is not typical of a 'R' stamp or 'NR' (Nuclear) stamp.Any old Timers out there that know what this is?