The 1965 Sanko Procyon PV-I and this model, the 1966 Sanko Procyon PV-III were, in their time, the classiest and most advanced derailleurs in the world. They had aluminium knuckles and parallelogram plates (Campagnolo was still bronze), used Simplex’s double sprung pivots (which, in the absence of a slant paralellogram, gave a superior change to Campagnolo’s single sprung pivot), had ergonomically placed adjustment screws, used a cunning mechanism which rotated the outer cable stop and the cable clamp in synch to stay aligned, had a pulley cage with an offset pivot and were beautifully made.
Europe, despite its confident assumption of superiority, had nothing to touch them - only the SunTour Competition was in the game, with a superior basic design, but less exotic execution. Unfortunately the expense of producing such a ground-breaking design allegedly bankrupted Sanko.
I believe that the difference between the Sanko Procyon PV-I and the PV-III was that the PV-III had the fancy variable rate spring and more ‘wrap’ on the pulley cage. I believe the PV-II was the same design concept manufactured in plastic - however I have never even seen a photograph (let alone an example) of this wondrous object.
Needless to say, even today Sanko Procyons are highly prized in Japan and virtually unknown in Europe.
In case you’re interested, Procyon is the seventh brightest star in the night sky and, in astrology, is said to portend wealth, fame and good fortune. It is also a fictional planet in the 1958 Brian Aldiss novel Non-Stop. I don’t know if the good folk at Sanko were devotees of astrology or of of classic British science fiction - but I rather hope it was the latter.
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