Unless I am mistaken, the original optional factory air conditioner on your unit would have been an Armstrong Bay Breeze unit. If my information is correct, Airstream used these until Armstrong abandonned the RV market sometime in the 1970s.
Is it possible that you have an evaporative cooler rather than an Air Conditioner? It wasn't exactly uncommon for Airstreams of this period that were originally sold in the Southwest to have evaporative coolers rather than Air Conditioners.
Summer Air might be a model name for a major manufacturer's RV air conditioner. Is there a plate anywhere on the interior shroud that might give the manufacturer's name? This could even be a model name for one of Armstrong's Bay Breeze Air Conditioners.
Good luck with your investigation!
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)