The two companies aren't the same. The motorcycle S&S company is best known for their carbs, and has been around since the 50's.
The S&S that built your frame was an automotive header manufacturer that started out in Newport beach in the 60's.
They got into bmx bikes in the late 70's until maybe 83.
They moved to anaheim in the 80's, and went bankrupt in the late 90's, I think.
They made me a few custom headers when I used to race my 75 honda civic in the scca in the early 90's.