Parkerizing

Parkerizing is a common metal finish which also can be broken down into two subcategories. One being Manganese Phosphate and the other being Zinc Phosphate. Actually a very old finish that did not take off immediately. By WWII this finish had came into its own. For fun Google which type was used on "name your make & model" and you will get a lot varying and almost no definitive answers.

A great finish for durability, and unfortunately outside of military restorations gets passed by more these days. It's unfortunate as a Manganese finish which is hot blued right after phosphating is a great looking and ultra durable finish.

Extra credit trivia: Which automotive engine part was Manganese Phosphated for years before technology changed? Answer in the next paragraph.

Parkerizing is most commonly seen on military firearms of the WWII era, where both handguns and rifles were parked. Answer: camshafts, for it's excellent anti-gall and oil holding properties during break-in.

Coincidentally, that oil holding benefit we talked about above would very easily lead into a massive discussion as to why those old military arms had those cool greenish colors.