This cover illustration from the June 1927 issue of Radio News is more or less self-explanatory. But unfortunately, there’s little in the way of explanation given in the magazine, and I’m not aware of this form of video recording ever having been done in practice.
The early mechanical television signals were indeed, sent over the audio channel of AM broadcast stations, so it’s not far fetched to think that the audio could be recorded on a phonograph disk. I think the main problem would be the frequency response of the disk recording. As far as I know, the upper frequency limit for 78 RPM records, especially during that era, was around 5 kHz. I doubt if much video could be packed into 5 kHz bandwidth.
The magazine mentions, with no technical detail, only that John L. Baird was then working on the system, which he called “Phonoscope.”