This document, from August 1933, is the earliest Fichtel & Sachs patent for a derailleur that I have been able to find. Weirdly it is a French patent and appears to predate any relevant German equivalents.

The derailleur shown is noticeably more basic than production models. Also the chain guide above the chainstay (to deal with chain slack from the back-pedal brake) is unlike any production version that I have ever seen.

• Publisher: L’Institut national de la propriété industrielle
• Inventor: Fichtel & Sachs A.G.
• Country: France
• Date: Priority date August 1933
• Derailleur brands: Fichtel & Sachs
• Derailleurs: Fichtel & Sachs