Hello there, I was born and raised (and still live) in teh Sunset. I am 45 and went to Lawton, Hoover Junior High, and Lowell. My recollection of SDI was that it was a very loose group of young adolescent kids in the 70's(mostly Irish)who never really did any harm. They just made their presence known by swilling beer in the playgrounds on Friday and Saturday and then tagging Park and Rec buildings with their faux-cool S.D.I. moniker. The context in which these kids felt their identities threatened came about during the "Black Power" and "Viva La Raza" era as well as a huge influx of Filipino boys (prefering polyester print shirts opened down three or four buttons, very tight polyester pants, and platform shoes plush green the obligatory army fatigue jackets!).
There had always been lots of Chinese but in the 70's it became harder for any kid who had grown up in the Sunset not to feel like an outsider on his own block. Previous generations of immigrants had mixed readily and happily(my 60's elementary school photos read like a United Nations who's who). No one cared what your ethnic background was like. No one understood whose parents "had money" or not. Homemade cookies and sweets tasted good whatever land they were from. We all went into each others houses to play and we were introduced to "diversity" without having to have an International Day at school.
The difference by the 70's and thus the impetus for the adolescent stupidity of S.D.I. was that suddenly there was no place for them in the place they had grown up as their numbers dwindled and as society as a whole was championing everything and everyone but them.
OK---way more info than you asked for. Sorry if I bored you. Tell me about your interest in SDI. How did you remember that? Take care.