I found an under-chassis photo of a K-731 radio (in an eBay listing). The PTO is, in fact, under the chassis, as I had suspected. I also remembered (just now) that, in a thread I saw a while ago dealing with these radios (in a newsgroup, not in VK), there was a warning against moving or otherwise disturbing the lead dress or component placement in a small box under the chassis; if any of this is disturbed, the radio can be thrown far out of alignment, and it will take quite a while to restore it. That "small box" is probably, even likely, the PTO.
The warning makes sense and should be heeded, since the PTO controls the FM tuning. It is possible to throw the alignment so far off simply by moving a component inside the PTO slightly (as in inches or fractions thereof) that the radio's tuning range can and in many cases will be reduced at one end of the dial and expanded somewhat beyond 108 MHz at the other, or worse; the only way to restore the correct tuning range, of course, will be to realign the radio's front end.
I don't know how easy or how difficult the alignment procedure is for the K-731, and I darn well don't want to find out. My set, even after 52+ years, works and sounds great as it is, so I am keeping my hands the heck off the chassis unless I have to change a tube or perform some other service or maintenance procedure. In the meantime I will just enjoy the radio as it is. I see no reason to do anything to it at this point. Why mess with it when it is working so well? This radio pulls in lots of stations on both AM and FM, even in my near-fringe area for Cleveland FM, and sounds wonderful as well.
I don't know how many of these radios (besides mine) are still in use, but the ones which are still in active service are positive proof of the quality of the older Zenith AM-FM sets. Too darn bad the company went completely out of business by the 1990s.
The world lost what was one of the best radio and TV manufacturers then. We will never again (unfortunately) see the likes of the original Zenith Radio Corporation of Chicago.