Shari is correct, the numbers make it a:
1963 Ambassador 28'
Ohio | 28 3 T(D) NNN (I for Int'l trim)
California | 28 T(D)S S 0316 to 28 T(D)S S 0372
T=Rear bath Twin; D=Rear Bath Double
The link is bad, but the page is really at:
Vintage Airstream - '63 Ambassador