Metallic Gray Paint Code!
I got the answer from Rod at Airstream Customer Service (937-596-6111) within 1 hour of calling and leaving a message (at 3PM on a Friday afternoon) - these people are great!
The answer is:
1980 to 1987 Airstream motorhomes and trailers
DuPont Centari 44146-A
But, if you only need a touch up, a 12 oz spray can is available from Airstream for $15.53 (part # 360151).
Hope this is of help to someone else out there.