I think the "m" series was the later rendition of the Model X PA cabinets, it should be the smaller, bordered logo with the individual grill screens I believe.
or it might just be a black grill, can't really recall. as far as speaker recommendations, I can't really say, I'm not sure what the original speakers were at that time or if you would even want to use those for guitar or bass since they were mainly PA cabs.