Old Remington Shotgun Finish

For the Remington shotgun experts...

Several years ago, I acquired an 870 Wingmaster 16 Ga Shotgun (Serial# 589XXXW) that according to Remington was made in 1958. It is in very good condition with a very nice stock (a small hairline crack behind the receiver) and forearm. The trigger housing is blued/black and the shell tube and slide action are blued/black. The bolt is chrome/silver. However, the rest of the metal finish (receiver and barrel) is gray (looks like it may parkerized). There is some minor pitting on the left side of receiver (hand prints maybe).

Do any of you guys know anything about this finish?

Thanks.