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