Hi, I no longer have a Zeta but I know they were still being produced in the late '80s. I worked for a US dealer and mine went back to England in '88 I believe. The bearings were changed (to 5 bearing configuration?) and it was rewired with silver VDH. I don't know exactly when GB Tools stopped, but I think it was '90 or a little later.
Regards,
Regards,