I would have expected the derailleur world of the 1920s to be busier than it was. The flowering of activity and invention in 1910 - 1914, killed by the First World War, was not properly repeated until the 1930s. However, there were two notable developments:
As with the previous two decades, it is striking that every derailleur system shown is French.
Browse the derailleurs from the 1920s...
Where you see a red link, this is for a derailleur model which I do not (yet?) have in my collection - but for which I do have some kind of real and relevant documentation.