This is a rare set, no doubt about it. Looks to me like early to mid-70s, not sixties (see my note below). The slider operated the color and other controls ordinarily adjusted by front-panel knobs on other sets. One positioned the slider over the function he/she wanted to adjust, pushed it in to engage the shaft, then turned the small knob on the slider to adjust the control and released it. (kc8adu had the correct name for this system, which I had forgotten--thanks, kc, for jogging my memory.)
I think Zenith only had this model in its lineup for a year or two, before returning to the traditional knob controls behind a panel door. I only saw a picture of the front panel of one of these sets, and that was in a Zenith service manual about 30-35 years ago. This was a great idea, but Zenith's and others' idea of putting all controls on the remote and, years later, in onscreen menus was and, IMO, is much, much better. Today's sets have few if any potentiometer controls, with just about every control that used to be on the front panel now on an onscreen menu. To tell you guys the honest truth, I prefer this system over the earlier one, as there are no pots to go bad, intermittent, etc. The onscreen controls are easier to adjust as well; no guessing, for example, as to when the tint control is at midrange. You see the exact position of every control right there on the screen.
Forty years? Where did the seller ever get the idea this set was anywhere near that old?
Zenith hadn't even thought of the original Chromacolor, let alone its successor, back then. That would place the set in the early '60s; Zenith and other manufacturers were making largely roundies at that time. Further, the single slider (Setronic) for all picture controls hadn't been thought of, by Zenith or anyone else to the best of my knowledge, back when roundies were popular.