Original Grappler schematic:

https://archive.org/details/manualsonline-id-6b56a0aa-4089-4658-a2c6-de559ed3a773/page/n17/mode/2up

The ProGrappler manual doesn't have schematics unfortunately.

https://archive.org/details/ProGrappler/mode/2up

I was doing a little bit of circuit tracing on the ProGrappler and two of the data lines are swapped I think. I'll have to check because I can't remember exactly.

According to my notes:

"I was looking at the prograppler dump and it looked really horrible, so I thought I'd do some checking on the board with a multimeter,
looks like they did a little bit of "protection" where they swapped d3/d4 and a2/a3 and a6/a7."