Others have already pointed out this is the tape in and that's the headphone jack. To the left is an effects send and return which is after the preamp but before the power amp (it's an in and an out where you'd plug in certain effects pedals). Far to the left is the instrument input. Depending on the specific signal you could probably route it through the effects return which bypasses the distortion of the preamp. I'd have to see exactly what the fuck they thought they were trying to accomplish.

The color of it makes me think of the old Peavey Rage 158 amps but they didn't have an effects loop or tape in. So I'm not exactly sure which amp this is. The far right knob is the master volume and the one left of the effects loop is reverb. That's not important but it's interesting to me.