Semi-s., you are indeed correct. Uncle Dave, in your gracious acknowledgement, you suppied the terms that I couldn't remember yesterday: transitive/intransitive.

I've heard "amn't," probably more often in movies and on TV than in the flesh, since it isn't common usage where I live. In my experience, I've heard the "m" vocalized pretty clearly; learning that the "m" is often silent among native speakers of dialects featuring this contraction, it's even more clear to me that "amn't" has evolved into "ain't," i.e., that "ain't" is a direct "descendant" of "amn't."