[DuMont]

I've finished the electronic restoration of the Philco 46-480 and while AM plays well, FM volume is very low even with volume turned to high.  The 7F8 tube is good, and I discovered a 4.7 uf instead of a 5 mf electrolytic in the 2nd IF stage.  Going to put the correct value in there, but what's really got me interested is a jack on the back of the chassis that says "FM Test Socket" and looks about the size of an iPod headphone jack.  I can find NO REFERENCE to this in the Philco service manual, Riders, or Sams.  Any idea what this is and what it's supposed to do?