Not too sure if there's a Police 870 that's not a magnum
The first 870 Police shotguns (And 870 Riot guns too) came out not long after the sporting version of the 870 was introduced. The Police guns had 18 1/8" barrels, the Riot guns had 20" barrels. They were marked Wingmaster on the receiver and were chambered for 2 3/4" shells.
Far as I know there is no 870 with a receiver stamped Police that is not a magnum, but the 870 Police pre-dates the magnum receiver.
http://www.thehighroad.org/archive/index.php/t-445488.html