[HAM] M-100 rca connection on "expression pedal"

[mtcasino]

It is a mono input used to play along with another source of music such as a
phonograph.

Mike

<snip>

I'd call it the volume pedal myself... anyway, (from the back) on the left
(opposite side from the linkage) there is an RCA jack, there's even a pug
(appears to have never been soldered), and there are no signs of any cable/
wires that might have been connected to this.
   
  Anybody know what it is for? (to save me from taking it apart)
   
  Thanks in advance
  Stainless/Sacramento