This is a mysterious and astounding derailleur. It is clearly branded Cyclo, both on the main arm and on the bearing cover plate of the guide pulley. But it is equally clearly a blatant copy of the Le Chemineau design, without any hint of Cyclo's signature helical mechanism.

Given that Joanny Panel of Le Chemineau and Albert Raimond of Cyclo were sworn enemies, and that Joanny Panel believed that Albert Raimond had stolen his design, this is hot stuff.

Unfortunately I cannot find any evidence of this derailleur in any printed material, so I can only guess at the date that it was created. I can think of three possibilities:

• It could date from the very start of the Cyclo story (1924?) - when Albert Raimond was working at RPF and looking for a reliable supply of derailleurs to replace the Le Chemineaus manufactured by the allegedly moody and unreliable Joanny Panel. The obvious answer was to develop a copy of the Chemineau and manufacture it yourself.
• It could date from the moment that the relevant patent expired. The key Le Chemineau patent (French Patent # 439,224) possibly expired 20 years after January 1912 - i.e. in January 1932. This is a year before the release date of the Cyclo Witmy - a budget model that was the first Cyclo pull-chain design that I have seen widely advertised. Perhaps this mystery model was a first try at a pull chain - but was not cheap enough to manufacture to make any sense.
• It could date from some time in the late 1940s - when Cyclo was trying to rediscover its place in the world after the trauma of World War II, and pull chain derailleurs ruled the world.

My personal bet would be on the first of these.

I would also note that this derailleur is constructed to a very high standard, with one exception, which is that the main tube, along which the derailleur slides, is bored wildly off centre. Could it be a prototype?

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• Derailleur brands: Cyclo
• Country: France
• Date of introduction: 1924?
• Date of this example: unknown
• Model no.: unknown
• Weight: 300g excluding mounting bracket and chain tension spring
• Maximum cog: 28 teeth?
• Total capacity: 28 teeth?
• Pulley centre to centre: 75mm
• Index compatibility: friction
• Chain width: 1/8”
• Logic: low normal
• B pivot: unsprung
• P pivot: sprung
• Materials: steel