A trailer of that age surely has some mods. If not, "all original" then it will be within 10% or so - empty. So, say 3000 pounds. But, you can see, I assume, from the description - have they increased the tank capacity with different (or added, i.e., grey-water) tanks. Added solar? Or something similar that would add more weight.
You may have found a really good buy - the 22-footers are desirable. And, a vintage Airstream opens up a sub-set of the Airstream world that can really be really be fun, and satisfying. The vintage crowd is very helpful to others (as I, and others on the Forum) am trying to be) and if you are at all "handy" with general repairs and maintenance, you can have many opportunities to both learn, and know more about your systems.
Fungus' idea to get a scale weight is a good one. There also might be a Forum member who lives nearby and might go do you a favor and give you a more expert opinion for... who knows?
Then of course, there's what you will pull it with. (Just a suggestion - "what should I pull it with?" can create a thread that can go off the rails, which might cause you to decide to keep that Bigfoot!)
Good luck on your adventure!
1976 Airstream Overlander International