True to a point, the mirror finish proofs were indeed struck at the San Fran mint. The 40% that the other posters were talking about, not only did they exhibit a business strike finish, but they were San Fran struck coins as well.
The Red Book is a great source of information for these types of questions and it would show that S-mint quarters were produced in both traditional clad as well as silver in both proof and business strike versions. The coin in question looks to me to be a silver business strike.
Actually the only copper-nickel clad coins (1776-1976 quarters and halves) from the SF mint were proofs, right? If I read my Red Book correctly.