The Shimano Skylark was Shimano’s take on the 1961 Simplex Juy Export 61. The Skylark has two sprung pivots, it is steel with no structural plastic parts, it has the pivot of the pulley cage offset from the axis of the pulleys and it works very well. Shimano called this combination of attributes their ‘Servo-Pantagraph’ design, and, despite this, it sold so well that they never looked back.
This example is the STO version of the mid 1970’s Shimano Skylark. It has very discrete branding (and a lot of holes). The STO versions of Shimano’s various derailleurs were always explained to me as being produced to sell to American bicycle manufacturers who wanted to hide the fact that the derailleurs that they were using were made in Japan. This explanation does not quite explain this particular gear - it is clearly manufactured for the Japanese home market - with a ‘backwards’ hanger plate and an extended gear protector on the pivot of the rear knuckle. Perhaps Japanese bicycle manufacturers would have preferred to fit European gears - and were just as keen to hide the origins of the humble (but effective) Skylark.