The other respondents to your question pretty well sorted it out for you. The "N" is a nitro powder proof test acceptance mark found on all German pistols of that era. And the plating is undoubtedly recent. The Germans plated very few guns. and they didn't use buffing wheels (which wipe out markings) to polish them.
"Nirosta" ia a Krupp trade mark for their stainless steel. Some manufacturers, notably Sauer, offered barrels made of Nirosta steel as an option. The stainless barrel addeded less that 10% to the cost of the gun. Surprisingly, very few people bought the stainless barrel.