Roughly 1970-72. I have a 1971 (B year code) 12B13C52 chassis. About half transistor half tube with one Zenith duramodule PCB with a DIP chroma demodulator IC on it.
The first part of this video shows how I deal with Zenith cataracts like that.
https://m.youtube.com/watch?v=F8Wfg1I1Z8Y