Yup, it's an old tranny amp. Marshall made them from 1976 to 1980. No idea if they sound any good or not.
I don't know if they're collectable or not, but sound wise I suspect you'd be better with a more recent MG. I had an old solid state Marshall in the mid 80s, and it wasn't anywhere near as good as the current MGs. Solid state amps have come a long way since the 70s (though I still prefer valves :D )