Got it...the check is in the mail!
I noticed on the registration form that each model needed to be described as "Entente or Central Power". Technically, the US entered the war as neither of those.
http://en.wikipedia.org/wiki/Allies_of_World_War_I
The U.S. entered the war as an "associated power", rather than a formal ally of France and the
United Kingdom, in order to avoid "foreign entanglements".[SUP]
[4]
[/SUP]
Now I just need to keep my planes in one piece between now and September.