Although I could find no mention of "knuckle knives" in the California penal code, section 21810 specifically states that possession of "metal knuckles" is "punishable by imprisonment in a county jail not exceeding one year". It doesn't say if it is a misdemeanor or a felony.
So, if a knife has "metal knuckles" incorporated into it's design, yes, the simple fact that it has "metal knuckles" attached to it would make it illegal to POSSESS such a knife, even in your home.
It's also illegal to sell, manufacture, trade, or give away metal knuckles in California. Clearly California law has zero tolerance for metal knuckles and takes a very dim view of anyone who possesses them for any reason.