Remington will build to order if enough units of a given configuration are ordered at one time. To me that looks like a Riot configuration (20" barrel, 3 shot extension). I'd be interested to know if it was a CYL bore barrel. The wood looks to me to be Wingmaster wood, the usual Police or Riot wood is a non-checkered stock and a grooved short or LE length fore-arm.
Remington stopped routinely turning out blued Police guns a few years back, from current indications they're all phosphate finished now. There are a few that turn up from time to time still, as 'new old stock' or LE turn-ins. Blued Police guns were pretty common not that long ago, though. I have one, manufactured in 1987 according to Remington (I checked the serial number), with the receiver marked Remington 870 Police Magnum. It has an 18" (not 18.5") ImpCyl bead sight barrel and a 2-shot magazine extension that protrudes half an inch beyond the muzzle, as is typical for that arrangement.
I got it from a fellow THR member about three years ago for $275 IIRC. It's in very good condition, both wood and metal. It has a chrome bolt and a blued FlexiTab lifter. It has an aluminum trigger plate, with a shinier finish on it than the gun in your pictures. It has sling swivel studs from the factory, and a black rubber open-V pattern recoil pad, typical of that time frame. And mine has the expected uncheckered stock and ribbed short forearm.
I haven't seen everything Remington ever built, by any means. And that might be a limited run special order shotgun, I don't know. but I have no use for collector pieces or collector attitudes, I'm interested in working guns that are shooters, not wallhangers that have to be cosseted and worried about- and paid three or four prices for. There used to be an old gun show saying that reflects my attitude- "Collector's item? Good- just put it in those big cans out back, and the collectors will be here in the morning to collect it."
In short, I don't know if it is what the would-be seller says it is or not. I don't really care, it would be a matter of curiosity to me, but certainly not anything I'd pay that price to get when good LE trade-in 870s are available for far less $$.
If you can read the serial number, or if the seller will supply it, you can call Remington and they will tell you what configuration that serial number was in when it left the factory...
fwiw,
lpl