Here are my other guesses:
2. Your comment that the only airline with a nonstop did not also offer direct flights means the nonstop airline did not also have a hub in DEN or DFW... which points toward Pan Am, with 727s.
13: Republic, 727-200
18: United, with DC-8-61s / 71s.
25: Heussler Air Service, with a Lear Jet via ERI.