Airstream's FAQ page is useful for a number of Airstream issues: http://www.airstream.com/product_lin...rvice/faq.html
There is a fairly long PDF document for post-1980 VINs and it looks like your explanation is there.
1ST = Airstream Travel Trailer
G = Center bath, roadside
E = Excella
A = Semi-monocoque, 96" narrow body etc.
U = 34' to less than 35' (by convention, a 34-footer)
3 = triple axle
6 = check sum digit -- see the long explanation at link
L = 1990
J = Jackson Center manufacture
507334 = Sequential production number (does not easily translate to actual units manufactured and this info is hard to come by -- trade secrets y'know)
Enjoy your baby! Photos always welcome.