Gestalt suggested Cryptocephalus, and it went unambiguously to C. sanguinicollis sanguinicollis in the key from the 1968 revision of Cryptocephalus by White(1). Also, is in good agreement with prior BG posts for this (sub)species, and the Calif. Beetles Database link here indicates records from San Luis Obispo County.

Unfortunately, I don't have field info on what plant this was found on (someone in our field trip group showed it to me in hand, and I don't know the substrate where they found it). But the link above lists host plants including willows, blackberries, roses, and wild licorice, some or all of which were certainly present in the area.