The Suntour Competition can make a strong claim to be the first truly modern derailleur. It has a slant parallelogram, it has three dimensional cast knuckles, it has logically placed, clearly labelled, adjustment screws, it fits a Campagnolo end and has an adjustment screw, but most of all it is a wee jewel with the simple composed ‘less-is-more’ look that makes it identifiably the ancestor of today’s Shimano Dura-Ace or Campagnolo Record Carbon.
This example is a twin cable version. The parallelogram has no spring, one cable pulls one way, the other the other way. The outboard cable has it snipple at the derailleur end, and is presumably clamped at the lever. The inner cable is routed exactly as on the single cable ‘normal’ version of the Competition. It is possible that one very long cable loops around through the lever - I simply don’t know.
The 1966 SunTour Competition Twin preceded the Shimano Archery-W, and so is the first of the many twin cable Japanese designs. Unlike Shimano’s designs, which were marketed as commuter products with a twist grip control, the Competition Twin appears to have been marketed as a product for drop handlebar sports and touring bicycles. There is a Bridgestone touring bike with what looks like a twin cable SunTour Grand Prix shown a Japanese web site. SunTour would have been extremely familiar with the twin cable Cyclo touring gears, and would have considered them to have been premium touring products - and these twin cable parallelogram derailleurs may have been SunTour’s attempt to update the concept.