<geek>
OK, I'm a graphic designer and a complete type geek. So please keep this in mind as you read this post.
I'd been looking at Osprey's Sopwith Triplane Fighters of WWI and there was something that bugged me about their renditions of the Triplanes (not including the fact that they painted most of them in PC10 instead of PC12). It was the font that they used for the names on the craft ( A. W. Carter, Black Prince, Black Maria, etc. ). For some reason the typeface looked too perfect, geometric, and modern... and it didn't make sense in an era where those typefaces would have been painted by hand. It looked like the adhesive vinyl appliques that you'd put on a vehicle in the present day. It just didn't fit.
And I finally put my finger on what it was this morning: the typeface they used in that book (and on the replicas I've seen) is Futura, a beautiful, modernist typeface that was designed in... get this... 1927.
http://en.wikipedia.org/wiki/Futura_(typeface)
Not that this is a big deal, but I thought it interesting for those of you who strive for complete historical accuracy... don't forget that the lettering on those planes was painted by hand... and while sans-serif type had been around for a while, the perfect geometry of an ultramodern typeface like Futura, Gill Sans, Helvetica, Univers etc. came too late for the WWI era. Stinks that so many of the decals that come with our models are done that way...
So, what's right for type? Well, the German type geeks (printers) did have a bead on modern type (do a search for Akzidenz Grotesk, which looks a lot like Helvetica), but I would suggest that it's probably more accurate to look at the manufacturing indicia on the plane (Sopwith, Fok EIII, etc.) to get an idea of what the type would have been like...
</geek>