It is not anodizing, the part it not aluminum like AR lowers.The heat treatment of the metal will cause the anodizing to not adhere/stay in color. Hence the purple color. P7s had this issue as with certain batches of Bushmaster receivers.
This is what I meant. My german made HKs have this issue, the US made one is still black.Is this the patina of which you speak?
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As stated earlier, the hardening process of the metal prevents the HE finish from adhering to the metal. The metal is still protected, so it is purely cosmetic.