Hi Ray,
The selector was indeed the big black lump stuck on the faceplate above the neck pick up and it used to have an ivorene knob on it as I remember.The cover was stuck on the faceplate with some sort of plastic glue (I think they used a solvent to melt it on originally) probably Airfix plastic cement.
It was three position but could be bridged between the three main positions.Made weird noises as I remember but remember my memory is getting on a bit.Fifteen plus years since I touched any sort of guitar and that would have been my old hot rod Fender.Or the Bender Tattycaster that lives next door to the other broken down guitar in the closet.Like I said,I break things.
I'd love to restore it as so many of these have been stripped for their pick ups over the years.Just have to see if the scratchplate shows it's face when I go up in the loft (shudder) to sort through the junk up there.I still think the multicore was a birds nest.
Gretsch arrives this morning.