"GP" or "CP" marking on barrel, is this a military barrel?

[porkchop]

So I bought a 1917 without doing too much research beforehand...I know I know. Well my question is, has anyone seen these markings before? I can't find anything on google about them, I'm guessing this is some sort of commercial barrel, what are your thoughts?

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[Calif-Steve]

Gun Parts Co. barrel? Not GI for sure.

[Mk VII]

Looks like a British commercial proof mark.