A few premium Magnatone amps had a "Stereo" input jack placed between the two inputs for each channel. 1 While this might seem to be a jack that would bridge the two channels by inserting a mono instrument plug into the jack, this is not the case. This jack requires a stereo plug, and one of the two signals is bridged to the high gain input of one channel and the other signal goes to the high gain channel of the other. If you plug a mono instrument cable into the stereo jack, you'll only be using Channel 1. See the schematic below for a better understanding of this. For your guitar, you can simply bridge the channels with a short patch cable, or build a mono to stereo cable where by the two stereo signals are wired to the mono on the instrument end.
So why did Magnatone use a single stereo jack when most guitars and basses were mono jack instruments? For one, it was an inexpensive feature to fit a two channel amp with the stereo input. Perhaps it also had to do with the growing accordion market that Magnatone was actively pursuing (especially with all their OEM work for accordion resellers like Imperial, PANccordion, Titano, etc). Stereo outputs on guitars were available, but not too common. Magnatone fitted their top-end Mark X Barth designed guitar with a stereo output as did Rickenbacker with some of the Ric-O-Sound Capri series guitars. A couple of top-end Gibson and Gretsch guitars also made stereo outpuys an option.
If you dig schematics, you'll notice that the one above (M10A) doesn't employ the self-biasing method of bias that is commonly used on most 12AX7/7025 input stages. For a properly functioning tube, the grid must be "more negative" than the cathode (google 'bias' if you need more than that to refresh your memory). On this amp, Magnatone biases that first stage by referencing the cathode voltage from B+ via the 220k resistor and to Ground via the 470 cathode resistor. This keeps the cathode at a "more positive" voltage than Ground at all times, and the grid references Ground via the 4.7M, staying nearly at ground potential at all times (0 volts). the tube ends up about +3v on the cathode, and 0v at the grid, thus a nice healthy bias.
1: The 460 of 1961-1963 was so equipped, as were several of the Custom Series amps: M9, M10, M13, M14, and M15.