Graph Tech Ghost - Manual (Page 70)

70
To prevent critical connections from being plugged in the wrong
way, some pins have been clipped on the Hexpander, the 13-pin Jack
board, and the Acousti-Phonic. Corresponding holes are blocked on
the plug-in connectors so that they can only go together correctly.
In this diagram, the black squares indicate pins that are missing.
Missing Pins (keyed connections)
Acousti-phonic Hexpander