By the way, for those just going through this thread, here is a summary of the references and what they say.
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1. Life Magazine, August 16, 1943, p. 51, in the article "P-38":
https://books.google.com/books?id=RVAEAAAAMBAJ&lpg=PA51&dq=gabelschwanz%20teufel&pg=PA51#v=onepage&q=gabelschwanz%20teufel&f=false"Into that camp the military police brought a disheveled German flier who was mumbling hysterically and repeating something about 'der Gabelschwanz Teufel'. An interpreter was called, and he had the translation quickly: 'the fork-tailed devil'. The German was talking about the P-38."
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2. From "Jungvolk: The Story of a Boy Defending Hitler's Reich," by Wilhelm R. Gehlen (flak crew member)
"There were four of them, the new American fighter bombers, P-38s or Lightnings made by Lockheed, which could also be fitted with drop tanks and used as a long-range escort. . . . We had given them the name “Gabelschwanz Teufel” (fork-tailed devil) on our identification chart a few weeks ago, but this was the first time we had seen them for real. We had been told that their range was insufficient to reach us in Germany, but these devils had drop tanks."
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3. From "A Higher Call" -- Author: "What you’ll read in the following pages is built around four years of interviews with Charlie and Franz and four years of research, on and off." Franz Stigler was a 109 pilot, and here is what he relates:
"THREE DAYS LATER, APRIL 16, 1943 . . .From Olympus, the controllers radioed the flight to alert them that P-38 fighters had been sighted above the Gulf of Palermo. Franz had never seen a P-38, but he had heard the name the boys in Africa gave the new American fighter—“ the Fork-Tailed Devil.”
. . .
"Franz liked the idea of pursuing “the herd,” as the bombers were called, instead of “the Fork-Tailed Devils.”
. . .
"He saw green silhouettes just two thousand feet below him. At sixteen thousand feet they motored in the opposite direction, toward Africa. Franz’s eyes went wide. Each fighter had two engines, one attached to each large wing. The engines’ booms extended back like fork blades connecting to a small tail. They were P-38s, ten of them, the Fork-Tailed Devils of the 82nd Fighter Group. The Americans called their planes “Lightnings.”
. . .
"When Franz and Willi landed at Trapani, they hurried to fill out their victory claims in the operations shack. Willi claimed two P-38s and Franz one. Willi was cheerful because they had chased away an entire flight of Fork-Tailed Devils . . ."