Did you try the California Secretary of State record of businesses?
They should be able to tell you although there is likely to be a fee
for looking up defunct or acquired company names. Good luck in your
sleuthing - this kind of thing was part of what I did once and it can
be challenging. But you at least know the area code - even though it
likely applies to the time before the code was split. That should be
a big help to at least giving you a starting date - date of split of