In September 1908, René-Jules-Alphonse Delacroix took out a Belgian patent for this design for a 2 speed derailleur system. In September 1909 Boizot patented this same design in France (crediting René-Jules-Alphonse Delacroix).
Frank Berto notes that line 24 on scan 1 of this document includes the term ‘un organe dérailleur’. He suggests that this may be the earliest use of the word ‘derailleur’ in a cycling context. Of such things history is made.
Another patent, taken out by René-Jules-Alphonse Delacroix, covers a transmission including an oval chainwheel and chain tensioner. This was also produced by Boizot.