"Back in the day" a Police shotgun was nothing more than a sporting shotgun with a shorter, usually cylinder bore barrel, and a plain un-checkered stock with an oil finish.
Remington also used a shorter fore end and were just the standard Wingmaster with a slightly less polished blue job. These guns were often stamped with Wingmaster receiver marks.
Things changed when the police started demanding guns with special features, and Remington brought out the budget 870 known as the Express.
In order to keep these versions distinct, Remington began giving Police guns Police receiver stamps, and began offering a wide variety of special Police options.
The only actual difference between todays Police gun and the older Wingmaster Police are the cosmetic changes to todays guns, the special options, and the markings.
The modern Police 870P does use a couple of stronger springs.
Because todays Police gun is more carefully inspected and built, quality is still equivalent to the old Wingmaster Police.
A direct comparison between similar guns might help.
An older 870 Wingmaster Police:
Un-checkered walnut stock with oil finish.
Satin blued finish.
Chrome plated bolt.
18" barrel, Cylinder Bore, bead front sight mounted into the barrel.
Short fore end with grooves.
A modern 870P Police gun of the same type:
Un-checkered walnut stock, more likely a synthetic.
Parkerized finish.
Black finish bolt.
18" barrel, Improved Cylinder Bore, bead sight mounted into a ramp base that's brazed to the barrel.
Short fore end, grooved, more likely synthetic.
Heavy duty magazine spring, carrier dog (lifter) spring, and trigger-sear spring.
The modern Flex-Tab bolt and lifter assembly that allows forcing the bolt open if a shell is jammed on top of the lifter.