Do you have a copy of Fredrick's obituary? There is an obituary volunteer for the Mendocino Beacon at freepages.genealogy.rootsweb.ancestry.com/~obitl/volca.html
Scroll down to Mendocino Co.
I don't remember if CADI Mom told you where to find his death certificate. You'll find their web site at www.co.mendocino.ca.us/acr/recorder.htm
Have you looked for Fredrick's birth certificate in San Francisco County? That sounds like a reasonable assumption considering the children were living there in 1870. Successful Searching!