Help with Phono - RCA Victor 18t AM Radio

[rodmonster]

I bought a 1940 RCA Victor AM / Shortwave Radio today. Model 18t.

http://www.radiomuseum.org/r/rca_18t.html

Pics are below.
Ok - Im cleaning it up and it runs pretty well. Decent sound.. Stations work.. etc..
BUT...
For the life of me i can figure out how to use the Television/Phonograph audio jack. There is no button on the front to indicate its use. The jack on the back looks like a standard RCA - but in reading up online i think it may have used a quarter inch RCA jack instead. I have tried a standard RCA and a quarter inch adaptor. Nothing happens.
I have put it in every setting on the front - and it still just plays the radio...
Ideas? I would love to be able to plug in an FM radio or Phonograph to it...

Please Advise...