Brit Mk2 usually have a makers mark (in this case Joseph Sankey and Sons) a date and a batch code for the steel which made the helmet....and then some idiot came along and positioned the strap lugs right on top...or the other way around...out-of-position stamping more likely. Sometimes Civil-used Mk2 were stamped with other stuff like wearer's initials, company names etc. My money is your "R" is part of the steel batch no.