monstercab question

[Batsinthebelltower]

hey guys  ;  what is the black jack socket on the back of the monstercab for ; it lines up behind the load ohmage selector switch , is it a "thru" ?  :dunno:

All other connections are obvious and labelled , but this one isnt - ive had it in my rack so long , i never got to have a good look at the rear ports, and just removed it into a 4u , so thats why im wondering    :banana-upsidedown:

[MarshallJMP]

That's a headphone output.