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I'm not an expert on older models but I looked on the Airstream web site and don't see a 28 ft. trailer listed for 1970. It does appear International is a trim level on certain models. Airstream has not been consistent with model names and trim names over the years. Take a look at the link below
The document archives on the Airstream web site are not complete so there's a chance the above document does not cover all trailers.
You may want to contact Airstream directly with your VIN to determine the precise trailer type. I think the old VINs have a different number of characters (letters and numbers) compared to modern VINs.
Looking at the pictures of your trailer it appears you have quite a bit of work to do. Best wishes on the project.