One possibility of the origin of those pickups could be the former "Nisshin Onpa" division which used the Maxon brandname. In 1981, Fujigen Gakki acquired the entire Pickup Division from Nisshin Onpa.
Since the California pickups are PAF clones (vs. the hot XR-7), Maxon had a pickup called "Dry Z" closely resembling the original PAFs, which was developed by Mr. Jun Takano (Music Land Key, later Kandashokai and K&T) in collaboration with Mr. Ishimaru (head of the pickups division at Nisshin Onpa) for Maxon during the Nisshin Onpa times already. They were used in various versions by Greco for their guitars.
So my guess would be Fujigen Gakki in Matsumoto-shi as the manufacturer of the California pickups at that time.
Ralf