Hello from Colorado: Welcome to the vintage Airstream hobby. I think 1977 was the last year of the Overlander 27'. I think Airstream consolidated the model offerings to 25' Trade Wind/Caravanner, then a 28' Ambassador, and then the 31
Sovereign in 1978. Three foot increments in length instead of two foot. I have not ever seen mention of a 1978 Overlander.
Anyway, thanks for the as found photos. I see your trailer doesn't have that goofy wall mounted oven, and doesn't have that goofy "shelf" between the fridge top and the overhead lockers. I see yours is a double bed model.
You might like to start a "project thread" in the Knowledge Base, Airstream Trailers, then Overlander, and then start one in the 75 Overlander year. Just title your thread "1977 Overlander Renovation" or some such thing. Folks like me like to see other ideas and progress. Folks like me sometimes have some useful information that might help you, although rare. Folks interested in Overlanders typically visit this area of the Forums.
Here is a photo of my 75 Overlander "as found". I am doing a significant renovation and refresh, but not a total gut job or shell off "full monte". My trailer didn't need it I figured. It was in pretty good shape. Maybe you would like to review my project thread since you now have an Overlander too.
David
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WBCCI #8607 VAC Region 11
KnowledgeBase trailer renovation threads: 69 Globetrotter, 76 Sovereign, 75 Overlander, 66 Trade Wind Such fun !