Shimano Positron Preselect (PPS):
The first indexed gearing system to appear on a significant number of bikes – the first version in 1975 used two pull cables, the second in 1976 (this one) used a semi-rigid push-pull cable. The parallelogram is not spring-loaded, and is held in position by a spring-loaded ball on a notched arm.
The problem was that Shimano thought that indexing would only be for non-enthusiasts, so it was introduced as a cheap derailleur – the weight and build quality didn’t do it any favours, and it was dropped in 1982.
The 105 groupset has been Shimano’s mid-range road groupset for over 20 years – this is one of the first, from about 1983. It has the horizontal parallelogram, as Suntour’s patent on the sloping paralelogram had not yet lapsed.l
One of Shimano’s odder fads – trying to convince everyone that components had to be aerodynamic. Pointless, of course, as the air resistance from the frame and rider is far larger than that from a little derailleur 😉
The AX has some clever touches, though. As the cable pulls it into the wheel, an arm notches around a stepped ramp. This indexes the derailleur, and also moves it down for the bigger cogs. This was the last derailleur Shimano made before Suntour’s angled parallelogram patent lapsed – as soon as they could, they dropped this fiddly bodge and went to angled parallelograms like everyone else.