Thanks so much for the reply! It's from an unpublished portion of Hemingway's posthumously published novel The Garden of Eden. Only about half of the manuscript was ever published, and I'm currently writing a book about the full manuscript. (For what it's worth, there are still a few unpublished stories--some not too bad, though none of them great.)

The passage with these verses also mentions Ernst Udet, who flew a Fokker. It also alludes to the French (post-WWI) pilot Michael Detroyat and mentions Archibald MacLeish's brother Kenny, who was a pilot who died in the war.

I agree about the authorship of the lines. I don't think the verse is a Hemingway original. Hemingway knew WWI aces like Billy Bishop and Udet, but this sounds more like an RAF song. Hemingway covered the RAF as a journalist very briefly in WWII, and he knew a young Roald Dahl when he was in the RAF, but I doubt anyone would have been repeating an old WWI verse like that in 1944. That said, Hemingway was a pretty meticulous researcher and I almost never check a detail in his work without pinning it down in something significant (although when writing first drafts his memory could be a imperfect).

I could be that the verses are authentic but just so bad that no one else ever bothered to write them down.... If anyone can shed light on them I'd appreciate it!