Shimano MegaRange what type? Can anyone identify this Shimano derailleur?
That's Shimano's C050 rear derailleur. It is comparable to Atlus and Tourney in quality and I believe only came as a 7 speed, like yours.

I've attached the technical manual so you can see how to set it up properly. If you're looking to replace it, Shimano provided a lot of high end 7 speed rear mechs. For a mountain bike that would include things like exage, deore lx, and so on. The same time period for road and commuter bicycles would be exage, some rsx.