Remington 870 Finishes: Wingmaster Bluing, Parkerizing, Express Bluing
Synchronizor posted very useful picture on Remington 870 Forum:
They look similar, and both are conversion coatings, but they’re completely different chemically. Bluing forms a layer of black iron oxide, while Parkerizing forms a layer of metal phosphate (different metals can be used, I’m not sure what Remington uses). I have both matte blued and Parkerized parts for my 870, and there’s a slight, but noticeable difference in appearance between the two finishes.