ok, now I'm stumped ... equipment isn't a 727 or a 737; I don't think US Air operated the ex-Mohawk BAC 1-11s much farther south than DCA, the only carriers that I can recall running DC9s along the Atlantic coast were Delta and Eastern, and as far as those are concerned, I'm highly confident that jlemon would have mentioned the availability of First Class in seeking either of these as an answer (that's why I didn't offer PA)
so I guess we're looking for either a startup or a previously extant regional that was beginning to spread its wings ...