>>731
It's the 870 Special Purpose Marine Magnum, designed for use by the Coast Guard and Port Authority. Mossberg also makes competitive model to the 870, the "500 Mariner".
The 870 Marine Magnum is electroless plated with Nickel, while the 500 Mariner has the Mossberg Marinecote, which is a propriety formula made up of Nickel and Teflon that Mossberg keeps closely guarded (probably Elctroless Nickel).
MSRP for the 870 Marine Magnum is around $840, while the 500 Mariner MSRP is around $550.
Mossberg also makes the 590 "Mariner", also coated with Marinecote, but at a higher MSRP than the 500.
Both can be found cheaper used, as low as mid $600s for the 870 and mid $400s for the 500.