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Grossflugzeuge
 by Guenther Bock
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 written in  
 
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published by Vandenhoeck + Ruprecht
 
Goettingen, 1931, ISBN ????, 40 pages plus 47 photo pages 
 This is not a pure G38 book, Dornier Do X and H.P.42 are mentioned as well.
 
  
  Out-of-Print
 but you have good chances at
 
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