I never heard the term "compliance plate" before, but I suspect it refers to the NHTSA VIN number plate with the umpteen-digit VIN number. If I remember correctly that contains a line, "XXX certifies this vehicle complies with the Federal Motor Vehicle Safety Standards in effect as of date of manufacture", or something like that.
NHTSA came into existence with the National Traffic and Motor Vehicle Safety Act of
1966, three years after your Bambi was built.
So you don't have one. Hope this isn't a problem for your buyer.
Don't know if it would meet the buyer's legal requirements, but Airstream, Incl has a lot of historical data on their trailers on their web site, under the "service" tab
Document Archive - Airstream You may be able to find the information there.
Good luck.