I have a Marinecoat 590 and it's not stainless, but . . .
I visited a gun shop in the Sioux City area a few years back, with a huge used section. They had what was clearly (to me, at least) a stainless 18-1/2" barrel Mossberg. Looked very different from mine.
The marinecoat has something like a matte finish. The one I saw was smooth. You probably could've polished it up to be super shiny (not that you'd necessarily want to, just observing).
At the time, I didn't know enough about them, and wasn't in the market for one, so didn't ask many questions. However, a store worker did stroll by and tell me it was stainless, and rare. Made sense to me at the time, since the used price was higher than my 590 new.
I didn't know what unicorns were at the time, but in hindsight I may well have spotted one. FWIW.