IF it's like a 3 p'up Firebird, top position would be neck+middle p'up .....
middle position is all 3 p'ups
and down would be bridge+middle p'up.
One of the knobs would be a volume control for the middle p'up and you put the middle one in and out of things with its' volume control.
So if you turned the middle p'up off via it's volume control ..... then the selector would do exactly like a regular 2 p'up selector.
I had a 1954 pre-production prototype 'Fretless Wonder' which was what eventually became the Black Beauty line.
Sure wish I had that back so I could sell it.
I hated it 'cause of the tiny frets so I traded it for a Hagstrom Swede and a bag of white cross.