Shimano's, long and winding, history of indexing went through many steps, some forwards, some backwards. It may go something like this:

Twin Bowden cable systems

• In 1967 Shimano produced the first of the 'W' series derailleurs. These used indexing in the shifter (a twist grip) and two cables operating an unsprung, un-indexed, parallelogram derailleur. One cable pulled the derailleur towards higher gears, the other pulled towards lower gears.
• The first generation of Positron models appeared in 1975. Again there was indexing in the lever, and again there were two cables, but this time there was also indexing in the unsprung derailleur.

Single rigid 'piano wire' cable systems

• Next up, in 1978 was the Shimano Positron II followed by its many children. This replaced the twin Bowden cable set up with a single rigid, 'piano wire', cable that could push as well as pull. Both the lever and the derailleur were still indexed, and the derailleur was still unsprung. This basic (and much hated) design lived on for many years as a 'leisure cycling' system while Shimano continued to develop other systems that did not require a rigid cable.

Single Bowden cable systerms

• 1978 also saw Shimano's first attempt to get away from the 'piano wire' - the mysterious Positron III. This used an indexed lever, a conventional Bowden cable and an unsprung, un-indexed, rear derailleur that included an extra lever that caused the chain tension to move the derailleur in the direction of higher gears. It was a relatively low-end object, manufactured from pressed steel. It worked, but not well (at least not in the muck of a Scottish winter) - Professor Piehead would have declared it 'a partial success'.
• For 1981 Shimano released the Shimano Dura-Ace AX, which was almost the exact opposite of the Positron III. Beautifully realised in forged and silkily anodised aluminium, this used an un-indexed lever, a normal Bowden cable, and a derailleur with a sprung parallelogram - all very desireable and mainstream. But it had a subtle index mechanism at the b-knuckle. It was promising, but the indexing mechanism was not sufficiently meaty, robust or reliable.
• 1982 saw the first of the 'New Positron' series - the PV11. This was a development of the 'piano wire' designs, described above. However it replaced the rigid piano wire with a single conventional Bowden cable and a sprung parallelogram in the derailleur. The hope seemed to be that improvements in the general design of the chain, freehweel and derailleur would overcome the problems that had caused Shimano to adopt the 'piano wire' in the first place. This was despite the derailleur not having a slant parallelogram. Again Professor Piehead would not have been impressed.
• 1983 saw the second of the 'New Positron' design - the NP11. Like the Positron III, this used an indexed lever, a Bowden cable and an un-indexed derailleur. Unlike the Positron III it used a slightly odd, but relatively conventional, derailleur with a sprung parallelogram. Crucially, it was also the first Shimano derailleur to use a slant parallelogram.
• 1984 saw the third 'New Positron' - the NP21. This was similar to the NP11, but used a much more normal derailleur design, but with a 'Centeron' arm like a Shimano RS.
• And finally, in 1985 we arrive at Shimano Index System (SIS). This was launched on the extremely high-end Dura-Ace 7400 and also, possibly at the same time, on the rather low-end New Positron NP31. It was fundamentally very similar to the NP21, but used a floating 'Centeron' guide pulley rather than a 'Centeron' lever. This is the basic systerm we know and love today.

This is the final version of the revolutionary Shimano Positron (DG100). It has:

• A pulley cage with a promounced 'pusher plate' at the guide pulley.
• 'Center adjust' is written in a circle around the adjusting screw.
• A cable adjuster that is able to float to optimise cable alignment.

• Derailleur brands: Shimano
• Categories: Shimano - Positron indexing insanity
• Country: Japan
• Date of introduction: 1977
• Date of this example: 1977 (two letter date code BD)
• Model no.: DG100
• Weight: 320g
• Maximum cog: an unlikely 34 teeth according to Shimano
• Total capacity: also an unlikely 34 teeth according to Shimano
• Pulley centre to centre: 54mm
• Index compatibility: 5 speed
• Chain width: 3/32”
• Logic: none - unsprung parallelogram
• B pivot: sprung
• P pivot: sprung
• Materials: steel