This was the second incarnation of BSA’s unique ‘tube’ design. The cable nipple lives inside the pulley hub and the cable runs up the tube (around the smoothly radiused bend) and out by the adjustable cable outer stop. The cable is clamped at the lever. As the cable is pulled, the pulley hub slides along the tube, moving the chain (you can see this all explained in UK patent # 640,473). Chain tension is provided by a spring running along the chainstay and attaching at the eyelet close by the adjustable cable stop.
Fiendishly simple and beautifully made, but wasted on a single pulley design that was never going to change gear accurately.
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