After looking through the service info that leadlike kindly put together for me, I'm pretty sure there's no UHF on my Zenith. It looks like there's supposed to be a separate tuner assembly mounted in front of the regular one which I obviously don't have in my tv. The knob itself should have UHF channel numbers listed around the outer circumference of the knob, which I don't have either.
Here's what the whole thing looked like the first time I removed it from it's cabinet.
Oh, and that other number was bleeding over in the picture because I was holding an LED to light it just for that photo. Properly mounted in the tv, it shouldn't show but one channel number at a time. 13 is the only instance where it projects a number other than what is shown on the gold bezel, in this case 25.