This strange-looking object is not just a derailleur - it is also a Britsh national icon, or possibly a British nationalist icon. It clearly moves its pulley wheel by deforming a parallelogram with pivots at each corner, and it was clearly doing this a good five or six years before that Italian, Tullio Campagnolo, supposedly 'invented' the parallelogram derailleur and changed the derailleur world with his Campagnolo Gran Sport. So this humble, British, Constrictor design is clearly of historic importance - except that it isn't.

Unfortunately a bevvy of Frenchmen (the horror!) had already patented, often already manufactured and even sometimes already sold parallelogram derailleurs in the 1930s. So this Constrictor parallelogram design was nowhere near the first. And with its ungainly width, its single pulley and its restricted capacity, this Constrictor design was nowhere near the most influential. But it gets a gold star for trying.

I have only ever seen this derailleur offered with a 14-16-18 three speed freewheel - so I have rated its maximum sprocket as 18 teeth and its total capacity as 4 teeth.

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I believe that this is an early example of the Constrictor derailleur. It has:

• A distinctive hanger plate with a hole, rather than a slot, for the axle.
• Parallelogram 'arms' that are rounded 3D castings.

It shares these two features with the 1945 patent and my May 1946 instruction leaflet. This is not the case with my other examnple!

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• Derailleur brands: Constrictor
• Country: UK
• Date of introduction: 1945?
• Date of this example: unknown
• Model no.: unknown
• Weight: 239g
• Maximum cog: 18 teeth?
• Total capacity: 4 teeth?
• Pulley centre to centre: not applicable
• Index compatibility: friction
• Chain width: 1/8”
• Logic: low normal
• B pivot: sprung
• P pivot: none
• Materials: aluminium main body and hanger, steel pulley, pulley cage and parallelogram