X denotes 1967 model year, but production date and parts installed will have date codes from 1966,
typically 35th week was the change over for a new model year.
These were the first models to use a 6HS5 for high voltage regulation, a small part called a VDR should be replaced to prevent excessive HV that Zenith was famous for. Back in '79, I lost a CRT in a (more plain) Zenith after not addressing this issue
Fortunately I have a full-line poster from 1967, a great model year for Zenith
SC600 re mote.JPG
1967 Zenith color matrix.JPG