Hi I have an 870 marine Magnum XCS so it's a kind of an oddball,
I got it special order it took me a couple of months to get it back in 2007 I think it was in about a month or so after getting it I ordered some extra parts finding out that a few things like magazine tube spring was stainless steel and a couple other little things that made it different and so I ordered a few extra parts and one of the things that the representative on the phone cuz it was a phone order at the time said you know that you should get a trigger set because you never know so I did and I asked him for a police set, I'm not sure if that's what I got it is metal, and it was bought around 2008 so since I think they stopped doing plastic in the police models in the early 90s and then the composites were basically only in Express models, then what I have in my hand unused and stored properly for 10 plus years is a police trigger set?? Right??

XCS already has the milled extractor in it which is kind of nice and it's kind of hard to tell the composite trigger guard cuz it also has kind of A wrinkle coating over it the plastic on this was even coated let alone the metal has that double coating on it...
So what would you do it already has a mailed extractor would you just go in and replace the carrier dog spring, the sear spring, and the composite trigger set that's in there right now and move on and still keep the metal one in the background as a spare..
Or would you change it out make sure that you change even the springs in this unknown metal one so I'm still going to put brand new carrier dog spring and all that from the police model from a known police part...
So I guess two part question is there a proper way of identifying this? I'm thinking by deduction if they stop making composite for police after 1990 and I bought this in 2008 and it's a police model trigger set...
And if it was properly identified I guess even if it wasn't would you go ahead and swap it out since you had it in your hot little hand?

The only difference between a police trigger group and an Express or Wingmaster trigger group is the springs. Why are you worried about polymer parts ? They have been around a long time now with no issues. I have seen more broken aluminum trigger groups/plates than polymer ones (zero).
There are a slew of parts differences between a Marine Magnum and anything else, and even more than one Marine Magnum version for several parts. Do you still have the parts in the little bags with the part numbers on them?

I'm probably just going to change out the springs cuz I don't really have anything against polymer either and it's kind of cool because that XCS coating is even on the plastic so for example the trigger guard that's normally just smooth composite plastic that's kind of a wrinkle finish to it... Thanks for the feedback guys I appreciate it