An unknown randonneur with the Derossi system. Translation below.
Here's a bike which is equipped with the Derossi gear multiplier. It's a very special system, if not brilliant.
To sum up, it's a system that, by adding 2 reduction cogs, allows :- either to have enormous development without having a very large bottom bracket. (I assume chainwheel size, gigfy) or, on the contrary, to have a very small development
As in this case, (If not mistaken, author) to be able to quickly change the development of your machine by only changing the intermediate cog, fixed to the base tube (Chain stay? gigfy). This base tube also has an oblong hole that allows the axle of this extra pair of pinions to be moved. If this oblong hole was used to tension the chain, then it is likely that the change was made for the pair of intermediate sprockets and not just one. By removing just one bolt, you could then prepare your machine for a ride in Beauce or the Pyrenees.
I'll have to take the system apart to confirm these points.
This Derossi system was patented around 1927. There are no visible marks on the bike. It has been completely stripped by the previous owner. I didn't see any holes on the steering column to accommodate a brand plate. The Cyclo derailleur, which looks to be original from the welding of the lugs on the down tube, must date from the late 1920s or early 1930s (Cyclo derailleur specialists will be able to clarify or correct this if necessary). The rear light, on the other hand, looks more recent to me.