I wish I could but unfortunately, I have no idea of such things. Hopefully someone on here will though.

The current US Air Force spec is TO 1-1-4, Exterior Finishes, Insignia, and Marks Applicable to United States Air Force Aircraft. There's a copy here (5 MB PDF). For Navy and Marine Corps, it's MIL-STD-2161(AS).

However, USAAF (US Army Air Force) specs would date back to WWII or earlier and might be hard to find. There are probably WWII enthusiast web sites or news groups that could help with those.

Many thanks Gus!

I did actually mean USAF not USAAF.

That's a very interesting document, but given the level of detail elsewhere, it's surprising that the guides to the letterforms themselves are quite basic!

As you say there are specialist newsgroups - looks like military modelling and flight sim people might have leads...

You're welcome, Elliott. I did some more research this morning (anything is better than working), and found an earlier manual that was on the street in 1941, although the available copies are from the 1949 revision. The manual number is TO 07-1-1, "Aircraft Camouflage, Markings, and Insignia," and copies (US$15-$20) can be bought at:

http://www.eflightmanuals.com/detail/itemList.asp?cID=4528

http://www.esscoaircraft.com/product_p/16427.htm

There's a book titled "Usaaf Aircraft Markings and Camouflage 1941-1947" that's on Amazon, but it's US$58.

Wikipedia has an interesting article on WWII aircraft markings (the color bands and unit insignia), but it doesn't cover the lettering.

This site has a number of geometric aircraft fonts that look pretty authentic.