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Symmetric ® (SunTour) Shift Levers
These shifters are mainly seen on Japanese "road " bikes from the 1982 model year. They use an "aero" mount where the shifters attach just above the top tube, instead of on the sides of it. Most of them used a special proprietary braze-on mount that isn't interchangeable with anything else.
These shifters are a bit problematical. Like other "friction " shifters, the lever friction is regulated by the bolt the lever pivots around. If you tighten that bolt a bit, the derailer will more or less stay in position. If you tighten it too much, it will be hard to move the lever. .
Unfortunately, these levers are a fundamentally flawed design. SunTour built in a special cam mechanism to make the front derailer move slightly to "trim " its position as you shifted the rear. This was a good idea -- it is a wonder that nobody else has taken it up -- but poorly executed.
The automatic "trimming" mechanism generally only works in one direction. Which direction that will be depends on the relative friction settings of the front and rear levers. Theoretically there's a particular balance point where it works right, but in practice this never works.
