Welcome to the Airstream world, and more specifically the Argosy motorhome world. You've got one of the more rare motorhomes ever produced by Airstream. I search for a 20' for some time before my 24' came on the market at an excellent price. As for floor plans, you can see the original Airstream documents here: Airstream, Inc :: Specifications
There's also some serial number info here: Airstream, Inc :: Serial Numbers
It seems like you have a mystery on your hands, though, or else something super-rare. According to what's on Airstream's site (which may not be entirely complete) it looks like the only motorhome made as a 1974 model was a 24'. Also, I've never seen / heard of a 20' model that had a rear bedroom / front bath. If this is factory, then it's truly rare!
As for getting the panel you need, there's a couple of good sources: The Airstream factory and Inland RV. Keep your eyes on eBay for a service manual, that will be very helpful, too!