To my knowledge there are two types of Marine Magnum 870's, the regular and the rare Police model, though there are differences between the current MM and older models.
The Marine Magnum came out in 1992 I believe. At that time it actually had an 18" cylinder choked barrel. The newer ones have an 18.5" barrel. The older ones generally have powder residue on the mag extension, an easy way to tell, since the barrel is shorter than the extension. They also had aluminum trigger guards and machined extractors and ejectors. I'm not sure when things changed to plastic trigger guards and MIM parts.
The Police MM model came out in mid 2000 I believe. It has the machined extractor and ejector as well as other 870P parts, the rifle sights on an 18.5" cylinder choked barrel, but it doesn't have the "Police magnum" on the receiver like the parkerized 870P.
Remington confirms that the current 870 MM is based off the Express. I don't know if the original Express that came out in the late 80's had an aluminum trigger housing and machined parts, but I suspect they did. Perhaps when Remington changed their Express line to plastic and MIM, the MM did as well, since it was always based off the Express.
We need a Remington 870 guru.