Welcome to AK.
I've been interested in radio and electronics most of my life (I'm 50 years old; started fooling with this stuff 42[!] years ago), and am also a ham radio operator (first licensed in 1972). I have a collection of antique and vintage Zenith radios, among them a K-731 with the 7M07 chassis. I like the '731 a lot. As you noted, the K731 is a great-sounding antique radio, thanks to its two-way speaker system (5x7 oval speaker and 3-inch electrostatic tweeter) and 35C5 output stage. I live in an apartment building, so can't make a lot of noise, but I'd think this radio could blow the roof off the building
(figuratively speaking, of course) if I ever dared turn the volume all the way up.
Zenith was one of the best manufacturers of radios (and later televisions) from the company's inception in Chicago in 1918 (the name "Zenith" comes from the ham radio call letters, 9ZN, of one of its founders). The firm remained in Chicago from its founding until about the early '80s, when they began manufacturing their audio products in Korea. Their televisions soon followed, with the entire Zenith plant itself being moved to Korea in the latter part of the decade. The company was sold to GoldStar, a Korean electronics firm, recently, which rang the curtain down on Zenith for good. The Zenith lightning bolt, which from the company's inception until sometime last year was their trademark, was used even on GoldStar TVs until just a few months ago.
I for one was very sorry to see Zenith fade into oblivion this way, as I have liked their TVs, radios and audio gear for years. I've had several Zenith TVs over the years and still own one today, a 19" Sentry 2 table model from 1995 that still works as well as when it was new; the only thing really wrong with it, if one could call this a fault, is that the grayish paint on the cabinet is flaking off in spots. But the set still makes a great picture.
Zenith had a clock radio in the 1970s which had a small LED in the dial pointer, which was used as a tuning indicator that worked on AM and FM. This was Zenith's F472W "Circle of Sound" radio, which had the speaker mounted upside down so that the sound would radiate 360 degrees around the base of the set. (The sound from that inverted 5"[?] speaker was great.) This radio also was available without the clock. I had one of the clock models which had a broken function switch; the clock itself failed soon afterward (the clocks in these sets were cheaply made and apparently not meant to last very long
, as these were the old backlighted moving-tape clocks; the flip-type clocks used in other models of these radios were just as bad as far as reliability and longevity were concerned). I removed the clock and rewired the power circuit to use a push-button switch. The radio worked every bit as well without the clock as it did with it. I lost the set when I moved seven years ago; I wish now I had kept it, as it would have been a nice addition to my vintage/antique Zenith radio collection.
Some models of the K-731 had the CD (Civilian Defense) markings on the AM tuning dial; some did not. The sets made during the Cold War era (1953-63) did have these icons, so yours is probably pre-'63 vintage. My own '731 does not have the CD symbols on the AM dial, so that puts it probably somewhere in the late 1950s. Also, the K731 was offered for sale in two different cabinet styles: the Early American design (as yours is) and a style with short legs (reminiscent of console TVs of early-1960s vintage). Again, the cabinet style can help pin down the vintage of your set. K731s of 1950s vintage are in Early American style cabinets; later models (until 1965) came in the newer style cabinet with the short legs.
The K-731 was discontinued after 1965, when radio and TV manufacturers (including Zenith) slowly converted to injection-molded plastic cabinets and circuit-board (as opposed to point-to-point hand-wired) construction. The end of the K-731 was, IMHO, the end of an era, as these radios (not to mention the C-845, Zenith's other high-fidelity table model radio) represented a level of quality and sound excellence we will likely never see again in this age of outsourcing and high-speed automated assembly lines.
Enjoy your K-731. As I said, they (and all other vintage/antique Zeniths) are great radios, and are built to last. Hang on to yours as long as you can. They sure as anything don't make them like that anymore.
Kind regards,