Made from 1964 to 1969, the SG 250 was based on a Meguro design for this four-stroke 250cc single making 18 hp at 7000 rpm.   Meguro was the oldest Japanese motorcycle manufacturer (1909), when it merged with Kawasaki in 1963.  More info on Meguro models here.  Could not find any explanation for the “170” on the tank knee pads but one webpage imagined that it was a code in katakana for “Meguro”, using this Japanese syllabary for the transcription of words in a foreign language.  Not sure what it means, but it’s interesting.