Le Spirax RD and shifter
 

Hello :)

I saw this rear derailleur and shifter on local advertising site and had to buy it. Newer saw it before and didn't even know that it existed. Information on the web is quite scarce. Something on Disraeli gears and one thread on Tonton. Book "The dancing chain" has it on page 164.
Apparently it was invented in 50's and was a randonneur-type derailleur, capable shifting 6 cogs, 14-28 at the rear and had a chain wrap of 32. The shifter has an internal indexing plate with holes and a bearing ball that fits in the holes, thus indexing. The derailleur has a spiral shaped cutout on the axle that pivots the arm when shifting and by doing that it maintains constant chain tension.

Thats about all I know. Tell me more about it. What were the bikes originally equiped with it? Why it is rarely seen on 50's bikes?
And where should I attach the distal part of the cable? There is this bolt with the hole and looks like a small nail in the hole (last photo), I suppose this is the hole for cable?

Yes, I'm inclined to think the nail through the hole in the bolt is there to keep the bolt from becoming fugitive. Take the nail out and put the cable through said hole and... you're on your own now, good luck!

Honestly despite your several good photos I cannot make any sense out of that thing. It looks similar to Simplex Tour de France or Cyclo Benelux derailleurs but not quite. Those were good derailleurs, but they had as fatal flaw: if bent, they would get involved with the spokes, much to the detriment of the derailleur, spokes, and often the frame of the bike. And alas they tended to get bent a lot.

The problem is that if a bike with a derailleur falls over on the right side, all its weight goes onto the derailleur. A parallelogram derailleur simply deflects out of the way, as if you were shifting to a bigger cog, but if the mechanism relies on a rigid arm, as here (and the others that I mentioned, among others) the rigid arm bends. Not good.

What bikes did it come on? Probably none. Derailleurs were after market weird $**+ that manufacturers of bicycles rarely messed with. The first factory bike I know of that came standard from the factory with a derailleur was the Raleigh Lenton Gran Prix in 1958. Bike shops of the day were perfectly happy to configure a bike to the customer's wishes. That's where they sold high end funky stuff like derailleurs