I don't know the specifics of what grades of Parkers "file marks" were present from factory. That said, these marks are typical on English guns. Jack Rowe, an English gunsmith, shows how they are applied in his videos on Youtube. The one I remember was with he and Larry Potterfield showing how to tighten up an action. he then re-applied the file marks.
The marks are applied by wrapping a metal file with fine grit sandpaper and filing across the breech face with the file on edge. It is a cosmetic procedure. Certainly is not emprical proof that a gun has not been used much as Jack Rowe performed the same application in a few minutes on a gun which needed to be tightened.
As for Josh's explanation in the auction bill, I believe this is what he is describing. Whether it means anything as to useage, is likely more opinion and other variables should be taken into account for condition/useage.
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