Gentleman,as Gunman said these lines are found on guns from pre-1900, and because of that,more commonly on hammerguns.Also on guns that have had their C.C.hardening worn off and likely been subjected to a liberal amount of blood, sweat, and tears, which I assume has allowed a certain amount of corrosion to act on the material in the fold lines that Hugh referred to.These lines are observed on guns by all the makers including Purdeys etc. A product of the action forging methods of that time period.

Better to keep your mouth shut and be thought stupid,than open it and confirm.