Take a look under the playfield for hand layout pencil traces showing where to put the wires.
I had a engineering prototype sample Southern Belle that had the original pencil layout lines of all the wires, sockets, etc. showing the assembler where to put everything. The s/n showed it may have been a prototype game before the sample games were made.
I have never seen any other game with this. Maybe yours has the manual layout lines too?
pencil1 (resized).jpg