[Hallicrafters] Halli 40A...an extra jack-in-the-back?

[Zengmeiste at aol.com]

  So the 40A works fairly well, but reception is not great, due to huge COM 
ED xformers just outside the window and other (DSL-related?) interference, darn.

   I noticed an RCA jack on the back, just above the antenna lugs, doesn't 
look original and the manual mentions it not.  I've heard  some Halli's were 
modified to use an outboard device for improving reception...or   could it be a 
PHONO input?! I'd rather not blow anything up but I'm tempted to uhh plug a 
cord in and see what happens..
  Thought? Thanks and 73s, Terry