Lozenge pattern on triplanes- not really a 'goof.'
Remember this is a fictional story.
After Stachel saves and impresses the Red Baron,his squadron are given a pair of triplanes. At least that's the way I interpretted it, it might be different in the book.
Stachel's squadron is not a famous fighter wing like Von Richtofen's Flying Circus, but only a standard one, as is reflected by the lozenge pattern on all their aircraft. (This material was mass produced by German/Dutch textile companies at the time and the pattern became standard issue.)
Is it not, therefore, quite plausible that Stachel and Willy's triplanes would be fashioned the same as all the others in their squadron?
Well that's the way I see it.