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Yes you likely are missing the thumb wheel cover. I have a similar Avante only no picture adjustments next to the thumb wheels, and there is an inner cover under the door hiding the wheels.
Those thumb wheels ARE almost certainly pots.....You see in varactor tuners the variable tuned element is a varactor diode....A diode that changes it's junction capacitance in proportion to the (IIRC)current flowing through it....So basically they use a resistor bias network to set the tuning of the varactor diode....That resistor can be a single pot (in the case of cheap SS single knob continuously tuned portables), or it can be a group of pots that are swapped for one-another with a switch scheme such as in your Zenith and many other varactor tuned sets of that time.
The advantage of your Zenith is that you could set up a good number of exactly tuned presets in any order you desire...The drawback is that you have a bunch of pots and a big switch to clean.
As digital logic cheapened they replaced the pots with digital gating of varactor current and made fully electronic tuners....They were also able to integrate auto-fine tuning and the like easier.
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