[Arran]

I think that the television input was actually wireless on these sets, they didn't use an input jack and a cable. Philco had a series of so called wireless record player accessories at that time that had what would later be called a phonograph oscillator, it would transmit at low power somewhere on the AM broadcast band. The pushbuttons on these 1939-42 Philcos operated a bank of tubed circuits each tuned to the radio station of preference, my guess is that the "Television" button was tuned to whatever AM broadcast frequency the planned, but never marketed, television receiver accessory was supposed to send the sound over.
The good news is that you can buy a modern equivalent of one of these wireless record player or phonograph oscillators in kit form and transmit the contents of your MP3 player, tape deck, CD player, and pick it up on the radio without adding a jack, altering an circuitry, or drilling extra holes. There are several of these on the market but there are other people on here who would be better able to recommend which unit is best.
Regards
Arran