I will try and answer this the best that I can, I believe that you are referring to the 870 Express Super Mag, and the 870 Police Model(s)
I am not aware of a Police Magnum 870, The only factory 870's with a "Police" Sticker that I am aware of come with Chambers that will accept up to a 3 inch shell.
Where as the 870 Express Super Mag will except up to a 3-1/2 inch shell which are namely used for waterfowl hunting. The super mags generally come with a Brown wooden stock and fore-end with a 28 inch vent-rib barrel. Not to be used with Slugs in this configuration.
The 870 Police Standard model's generally have Black stock and fore-end synthetic or wood with 18 inch barrels suitable for shooting slugs or buck shot. You can get into a variety of other options such as pistol grips and so forth but these are the "main" differences between to the two.
This is not an Express. I have been told that the 870 Polce Magnums were first called 870 Magnums - but I am not sure. It has a 3 inch chamber and has police wood on it (stock and forend). It is blued with an 18 inch barrel and extended magazine tube.
The police model has metal parts that the regular 870 doesn't. The trigger guard is one example. There are a couple other areas as well but i can't remember them off the top of my head. Their reasoning is that the police model will be used much more and w/ harsher conditions so they have metal parts so it will stand up to the punishment better. They are pretty much the same except for those few areas and you really have to look to tell the difference. Pretty much all it does is add another $100 to the price for it to say "Police." They also might have a better or different parkerizing.
"What is the difference between a Remington Express and a Police Magnum?"
The Remington Police Magnum shotgun is built heavier to withstand the
abuse and pounding that police departments inflict on their gear. It has a
metal trigger assembly versus the Express's plastic assembly. The Police
Magnum also has heavier springs (7 lb. sear disconnect spring versus's the
Express's 5 lb. disconnect spring). The Police Magnum has a 22 inch
magazine spring versus a 16 inch magazine spring in the Express.
The Police gun comes with a heavier lifter spring. Each Police Magnum
comes with front and rear sling swivel studs. These are just some of the
differences in the two guns. Remember, you get exactly what you pay for.
Both models function the same and digest the same ammo, so if you are
inclined to inflict this type of punishment on your guns then consider a
Police Magnum. Otherwise the Express model will serve you very well.
I just picked up a Remington 870 magnum wingmaster
(that is what is says on the left side of the action).
It has rifle sights on it and a smooth bore barrel that says 2 3/4 shells and nothing about 3" shells.
If it has rifle sights and a smooth barrel then it is most likely primarily intended for slugs and buckshot. Does it mention a choke on the barrel? Mine has a fixed modified choke. IIRC, tighter than modified and you cannot use slugs.
And, if it doesn't say it can take 3" shells, it can't take 3" shells. 2 3/4" shells are all it can take. They are plenty powerful enough for deer.
Here's the current differences between the Remington 870 shotguns:
The Wingmaster and the Police are the same gun with different cosmetic finishes.
The Express is Remington's "budget" gun made to compete with Winchester and Mossberg's much cheaper to make, cast aluminum receivers and stamped internal guns.
Almost all 870 shotguns are now 3" Magnum models, and have been for some years.
All Remington shotguns have the same forged and milled steel receiver.
The Wingmaster Sporting gun has:
Aluminum trigger group.
A smooth, commercial grade polished blue finish.
Very smooth, no burrs inside or out.
A smooth, polished bore.
Walnut stock with better checkering, and Remington's "Bowling pin finish".
A chrome plated bolt.
Old style magazine retention assembly. (No dimples"
One piece barrel.
The 870 Police is basically a dull finish Wingmaster.
An aluminum trigger group.
The old style magazine retention system.
A much smoother finish inside and out.
A military-grade parkerized finish.
A polished bore.
A one piece barrel.
Walnut or synthetic stock, with a short police-length fore end.
Choices in different stocks, including Speed-feed, and others.
18" to 20" improved cylinder barrel, with a wide choice in sights, including rifle, ghost ring, and luminous.
Heavy-duty magazine spring.
Heavy-duty trigger-sear spring.
Sling swivel mounts.
No lockable safety.
No use of MIM parts, the extractor is milled.
The Express Model has:
A plastic trigger group.
The dimples in the mag tube and the new style magazine retention system.
A rougher finish inside and outside, with burrs left in place.
A rougher blue job.
A less polished bore.
A two piece barrel. (not 100% sure about this)
Hardwood or synthetic stock.
Some Metal Injection Molded (MIM) parts, like the extractor.
NOTE: It's possible Remington has discontinued the MIM extractor due to breakage problems.
Also, It's not clear at this point if Remington is still using the "J" lock lockable safety button on the Express and Wingmaster guns.
Plus the Police model has more QC checks, and my 870 Police has a much thicker, visibly thicker, barrel wall than the 870.
The Express does have a two piece barrel, you're right! The oxide finish on mine was worn badly and I gave it to Tweak (arf.com) to parkerize; the ring that goes on the foreend tube is brazed in place so the parkerizing did not take there. Had to use stove paint.