This review of the bicycle shows of the winter of 1929-30 makes a specific point that the derailleur transmission is now the dominant form on quality bikes. While it mentions all the usual suspects (Terrot, Chemineau, Boizot et al), it baldly states that ‘Le Cyclo featured prominently on most stands’. The age of the bicycle manufacturer with their proprietary derailleur has ended and the age of the universal fitting derailleur as a widely distributed component fitted by many different bicycle manufacturers has clearly begun.
To add to this general impression, this is one of the earliest mentions of Oscar Egg as a provider of derailleurs. His Super Champion derailleurs, supplied a components, went on to become the dominant racing derailleurs of the 1930s - mirroring Cyclo’s dominance in the touring world.
As an aside, the image shows a very workable (half-step plus granny) triple chainset and a long caged touring front derailleur.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)