49- among US carriers this would have to be Republic/Northwest, at whichever stage of the consolidation it was at this point in 1987; Phoenix is about the only destination on their route map that makes sense ... that posited, however, I'm now thinking this operator was more likely from south of the border -- perhaps Aero California, to La Paz
51- let's move CO to Dulles, and toss American into the mix there as well ... which leaves our list with a BWI vacancy that might have been filled by ... Piedmont?