Remington used a few different finishes, depending upon which version of the 870 it was, and when it was made. Polished blue, matte blue, parkerized, and electroless nickel, IIRC,
Call Remington and ask them. I did this with my 870P MAX. I got mine from a guy in Florida for $660 shipped when they first came out and cost $1500 new. I asked for a build date as well and it was only 6 months old. I still have the shotgun and its the longest time I have ever owned a firearm. I replaced the old school 80 lumen lightbulb that came with the shotgun and was considered high tech at that time with a 500 lumen LED kit from Battery Junction. I also sent the barrel to Vang Comps and had Hans Vang work his magic on the barrel. I carried it on duty and now that I'm retired it pulls guard duty between my bed and nightstand.