Anyone who has looked at German arms and militaria from WWII and the years prior will be familiar with the 3-letter (or number) code system used to obscure where production was taking place. The more common codes are well known, but there are hundreds of smaller manufacturing companies or job shops that are much more difficult to identify. Well, not anymore. Michael Heidler of Germany has a book listing every one of these codes with its associated company and location (hundreds of pages worth):
Herr Heidler’s email address is firstname.lastname@example.org, and if you drop him a line he will be happy to get a copy of his book on its way to you. It’s written in both English and German, and if you focus on collecting German arms and equipment it’s a great resource!