Lightsped,
Any 12 gauge 870 with a serial number ending in M is a magnum receiver gun. It can fire 3" magnum shells IF it also has a barrel with a 3" chamber. The early Express guns were labeled just Express, later it was changed to Express Magnum. The only difference in the older Express guns and Wingmasters are 1) the finish, a bead blasted blue on the Express vs. a polished blue on the Wingmaster; and 2) the furniture, which is walnut with Remington's famous 'bowling pin' finish on the Wingmaster, but in the Expresses, either synthetic, hardwood, or (in the really old Express guns) dull finished low grade walnut.
Later on in newer Express guns, MIM (cast) parts crept in in a couple of places (like the extractor), then the dimples in the magazine tube came along in the design changes that did away with the spring loaded magazine cap retainer in the barrel ring and the spring steel magazine spring retainer. All that was replaced by a plastic plug in the end of the magazine (held in place by the dimples) that served to both retain the magazine spring and the magazine cap.
And eventually along came the plastic trigger plate, then the detestable (self-) locking safety button.
That's how you tell old Expresses from newer ones. Older ones are a better buy if you are shopping for used 870s IMHO, as long as they have not been mangled by amateur gunbutchers at a kitchen table somewhere.
You will see a few 870 SP guns, the Special Purpose guns. I don't know which line these come off of, whether they are Express or Wingmaster. The last SP gun I saw had a plastic trigger plate but it was on the used rack and I don't know if it had been swapped out with another gun.
Police guns are now made on a separate assembly line for law enforcement. They are the Cadillac of 870s, if you are buying new. There are some differences over Wingmaster/Express guns- mostly stronger springs and beefed up shell stops in the 870P. Used, they often turn up carried a lot but shot not so much. If you can get beyond cosmetic wear they can be bargains- but not as often as they used to be. A lot more agencies are having their 870s refurbed these days rather than trading them in.
As a side note, you might encounter older Police and Riot guns just marked Wingmaster on the receiver. Tipoffs: an 18" CYL bead sighted barrel gun with a 'corncob' or short forearm and a plain stock is a Police gun of the old school (most likely 2 3/4" only), a 20" barrel set up th same way marks a Riot gun of the old school.
There are still some 870 Special Field guns out there- these are 21" VR barreled, straight or English-stocked 870s. Nice guns for upland use, but the barrels WILL NOT interchange with other 870s- the barrel ring is in a different place on the SF barrel.
There are some competition grade 870s out there, marked by features appreciated by clay busters. If you see 870TB or 870TC, you've found one of these.
And lastly there is the 870 Competition, a single shot trap gun set up with a gas recoil reducing system where the magzine normally lives. You won't find lots of those, only about 5000 were manufactured IIRC. That's a pretty small slice of the 10 million or so 870s out there...
I think I got 'em all, save perhaps for the military marked guns- don't know of any ordnance marked 870s in private hands but there might be a few. Oh, there's all the Johnny come lately green TACTICAL!!!!! guns, and other stuff like that. But the list above covers the major varieties.
hth,