I think you'll find these to be a common key through the RV industry and certainly within Airstream. I'm not sure which key it is on the ring I have here. The one I keep at my trailer is stamped so I can ID it. Sorry. Someone here will confirm it, I'm sure.
You should be able to order one.
I don't know how to include an attachment here but google Airstream Water fill key and you should find it. It says G248 was used for water fill in 1974.
Roger in NJ
" Democracy is the worst form of government. Except for all the rest"
Winston Churchill 1948
TAC - NJ 18