France's 50-franc airmail for 1936 is Scott C15. Scott also lists a variety, C15a. It says the burelé is reversed, but doesn't offer a picture. Below are the two stamps, with drawings from a 1982 Yvert.
France C15 normal burelé. Leans to left.
France C15a reversed burelé. Leans to right.
I just acquired the copy of Scott C15. Although it catalogs for less than C15a, it is much harder to find. Most copies I have seen in auctions are actually C15a.
This came from a Kelleher auction in the United States. That auction had SEVEN of these things, all described as C15. But five were actually C15a's.
Ed, that C15 is smashing, big hole in my collection, lost every bid I have made on this fellow. Was not aware of the reverse burelage.