870 MMs are plated, all the metal parts are plated, wouldn't surprise me if the reason for different parts numbers is that the MM parts are plated. MMs cost almost twice what plain ol' Express HDs do. Haven't asked Remington, dunno if they would even tell me but I suspect MMs and HDs are the same basic gun except for the plating and the rollmarked name on the receiver.
Gun gonna be on a boat on salt water? Get a MM, no question.
Gonna be in your bedroom closet mucho miles from the nearest beach? Get an Express HD. Spend the difference on ammo and by the time you shoot it up that new HD will be smoother than any MM, and you will be a lot better acquainted with the gun.
Better yet, find a Cinderella 870, take it home, give it a good deep cleaning, maybe replace the magazine spring, and shoot it till it's an old friend. Someone else did the breaking in for you, someone else paid the new-gun depreciation, you can reap the benefits and save a bunch of $$$ at the same time. LOTS of Cinderella 870s out there, police gun trade-ins, used sporting guns, lots of them needing homes out there if you will just look for one.
Good luck, stay safe,
lpl/nc