Express models are stamped "870 express magnum" on the reciever.
Police models are stamped "870 police magnum" on the reciever.
All Remington magnum recievers are steel.
If the gun you are looking at is $299, it is an express. IMO it is still a better gun than the Mossy. The P models are usually between $375 and $500 depending on the configuration. If the reciever does not have "POLICE" stamped on it, it is not a P model.
The main differences in the express and the P models are:
Express--alum trigger
Police--steel
Express--birch stock(ex syn)
Police--walnut(ex syn)
Express--solid rubber buttpad
Police--fancy Remington buttpad
Police guns are also truly parkerized while the exprees seems to have a bead-blasted, blued finish.
The P model also has a lighter barrel, a high visibility follwer, and your choice of rifle, ghost ring, or bead sights, and supposedly a smoothed up action.
Also both only come in 18" or 20" barrels. No 18.5".
I think the 870 is a better gun than the Mossy either way.