This SY derailleur is a rather well finished 'derivative' of an early 1970s Shimano Skylark. It is a classic steel mechanism with two sprung pivots and an offset pulley cage.
I have guessed its date at 1975, but it could be any time between about 1970 and 1985. I have no solid information about SY derailleurs at all.
The SY and Sun-Bow brands were both designed and manufactured by Yamashita Iron Works Ltd., but the quality of chrome plating and the accuracy of the pressings on this SY model seems to be much higher than on my Sun-Bow models. A mystery.