Why are you posting a link for a trigger group? Why is the polymer trigger group considered inferior to the cast metal/aluminum group? No lubricant necessary in the polymer areas. Polymer trigger has shown durability on par with the cast metal piece if not more. A red (7-8lb) sear spring and the 11-87 carrier latch spring (as the carrier dog spring) are the reasons one needs to have a "Police" 870? Buy $15 worth of springs and an extractor and make your own!!!
There is no difference between any breech bolt remington makes unless is has the marine magnum zinc finish, and I'm not even sure that's an actual thing. A breech bolt is a breech bolt, (flex tab vs old style not withstanding) The P model gets a machined steel extractor, others get MIM.
SS follower? in the mag tube? from the factory? Last I saw it was orange plastic... the mag tube spring is 4"ish longer from the factory on Police models.
The parkerized finish on the Police models is by far more interesting a quality than the "internals" ie 2 springs.
Everyone can have 870 Police internals if you put two springs in your shotgun...
hone the cylinder... has nothing to do with the trigger group or internals. 870 barrels are 870 barrels, they parkerize and finish the Police barrels differently than the Express... duh.