I have been staring at the schematics for the CPU board for a long time now. Months, if not years. I just noticed that there is a typo on page 3 of 3. They label both the send and return for the switch matrix as SWITCH ROWS.
The new version of the CPU board for Stern games (CPU Sound Board II) has a completely new design and properly labels the connectors.
This has absolutely no bearing on anything, but it just struck me as odd.
For the record, the top connector driven by the the swstb (switch strobe) signal should be SWITCH COLUMNS.
In other general rambling news, my procrastination with cables has paid off. I located the exact 2x20 2mm pitch connector with ribbon cable for the sbc2440 GPIO port on ebay! This brings me much joy.