A little closer to the second option, I think. A lot of parts will directly interchange. These were made (IIRC) in the late 80s/early 90s, and did cause some friction between Ruger and AMT, but I don't believe it actually went as far as a law suit.
It drops right into a Ruger stock, and fits like a glove...
Those are nice rifles from what I remember. They made really nice stuff back then. I knew a guy that had a cased set of consecutively serial numbered .357 Automags. The .44 Automag was my childhood dream gun thanks to Mack Bolan and the Executioner series of books.
Amt went thru several incarnations.AMT stood for Arcadia Machine and Tool out of california. They moved to a new location many years ago though I for get where. They also mad a 30 carbince semia uto pistol and some long slide 45 acps. If I remeber correctly the main complaint was incocnsistant quality control.