I would say if the title states it is a 77 then that is what you need to refer to it as when buying parts.
Airstream did not make tremendous changes between model years, unlike auto manufacturers. The shell for a 69 is almost the same as an 81. There were minor changes in tail light assemblies, door handles, refer vents, kitchen exaust systems, and battery boxes. but none of these changes should cause a problem when ordering parts. You just need to be sure to get a full description of the new part and be sure it sounds like the old one.
Unless you really study the 76/77 models you more than likely will not be able to tell the diffrence anyhow.
So, I would say you have a 77 and do so with confidence.