Go to www.bobjonesguns.com he deals only in German, and austrian guns. Give him a call or e-mail him that picture and ask about the mark.

I'm sure the K is for Krupp, and the letters STAHL is german for steel the "R JPP" preceding the word STAHL IMO, are the type of steel the barrel was made form. Most barrels made by Krupp in my experience were labled "Krupp-LuftStahl" Which translates to Krupp fluid steel.