Series 300 layout

[Os]

I've just purchased a Mark IIB Series 300 Coliseum head. It's wonderful but I'm lacking in information. This unit was built without the optional reverb, so the presence control is on the front in the far right knob position. On the back panel, where the presence control would otherwise have been located, is a 1/4" jack. Does anyone know what this does?

 

[Boogiebabies]

If It were me, I would move the footswitch jack there instad of under the chassis behind the tubes. We would have to see some guts to investigate.