I asked the seller if she knew what it was for. She says it rotates the ferrite antenna, so at least the circuit should be original.
But I guess that's what one of the other knobs used to be for? So most likely the string snapped or the mechanism broke and the extra knob was added.
Martin, do you know how this thing is supposed to work? I'm somehow of confused. It has only two knobs - one seems to be the source selector, so the other one is most likely the volume control. But then how do you rotate the antenna (which is obviously built to rotate, see the slit in the back and the flexible connection) or tune to a station (or is the thing in the middle of the scale another knob?)? The knobs can't have dual functions as they have marks that correspond to the lettering….
Edit: All right, I think I got it figured out. The seller sent me some additional pics. The antenna was originally adjusted by a piece of cardboard that protrudes from the slit on the back. No idea how that knob should adjust it, though. The antenna seems unaltered on the pics. Maybe it's for something else after all.
Well, if Martin says it is possibly to get a replacement cloth I might as well place a bid on it...