I guessing the good old reliable Colpitts. Cheap and simple and very stable at low frequencies. The tiny coil in the phono cart is probably working as an A-Modulatorer on the grid of the 12J5, whether or not they run RF thru the coil or not (probably not) so it's a moving coil cartridge as there's no iron inside it, at all. Zenith may just be using the 12J5 to bring the voltage up to a level where they can inject it to the IF stage and modulate it there, because there were some healthy paper caps on the grid and plate that I replaced with "Orange Drops", value for value. Unless they used a heck of a tank circuit, which they might have, but it looks like an RF choke rather than a coil under the 12J5, now I'm wondering if the 12J is an oscillator. Part of my brain expects to see a 1-2" diameter RF coil 3-4" long under there, but then a ferrite core coil could be tiny at a few mW's too. "I'M SOOO CONFUSED!"
I am going to understand this Zenith Cobra RF phono pickup or die trying.
I found a schematic of a Zenith Radio/ Phono on the Nostalga Air website. It is free and you can google it. The Radio I found wa a pretty high end peice. It has a transformer power supply and in addition to AM has both the present FM band and the old FM band.
The tube in the RF modulator is a 6J5. My guess is that since they had already designed this circuit they wanted to use it in your radio so they used the 12J5 to do the series heaters.
I redrew the circuit into what I think is a more understandable form.
I was assuming the pickup was capacitive since I seem to remember some mention of a metal vane attached to the stylus. Since you mention that the pickup is a coil and I have seen other references which seem to suggest that I am now assuming it is a coil. The Ryder schemo does not show that.
The Oscillator looks more Hartley-esq than Colpitts with the pickup coil and the other coil forming a tapped inductor.
The RF signal from this modulator is fed to the control grid of a receiver IF tube and the recovered audio is recovered from the screen grid of the same tube. The magic by which this happens is not explaned by Ryder.