Dave (DaveWM) --

The 8-tube version of the K731's chassis is 8C01/8C02, found in the Zenith radios you mentioned and also in the C/H/L845 series. The only difference that I am aware of between the latter chassis is that at least one (IIRC, the H845) has a dial light, whereas the others do not. One I know for certain that does not have a dial light is the C845. One other difference between the eight-tube Zeniths and the seven-tube versions is the eight-tube sets all had an RF amplifier tube, used for both AM and FM. The seven-tube K731 has a RF amp but it was used only for FM; however, these sets all had two IF stages and a limiter stage. In fact, the limiter stage works as a "third IF" amp in these radios, making the sets very selective.

I have the Early American version of that radio that I picked up an estate sale for five bucks. It works but needs redone, and the cabinet is kinda nasty (it was probably a kitchen radio) and will have to be refinished and the grille cloth replaced.

Five Zeniths
 

Way to go, now if only I could find an "elevator music" station.
Here is a group of the best AM-FM radios made in USA.
We have a duty to preserve the best in consumer electonics!

Dave 63

I have those same 3 on the left side, great sets indeed.

For AM I have a H615, another top notch zenith AA6