I have mapped the contacts from the encoder circuit board to a matrix.
My first attempt was incorrect in that I had counted the black wire A9 in my matrix on closer examination it is a ground wire and not directly connected to the keyboard membranes switch contacts.
I had seen it suggested to connect the keyboard encoder to a computer and then use “Note Pad” or a similar program to show the character that appears on each linked pair.
I elected to hook it to the “Retropie” and list what came up in the Controller setup screen. This proved to be a very effective way to chat the outputs, but it did require multiple resets to complete all the combinations.
The only key not proven this way was “A6 to B15” this pair aborted the setup and took me to the command prompt. I suspected this was “F4” which is the key marked to quite the setup routine.
Later I tested this using a keyboard test site I found and it was indeed the pair for F4. Had I located the keyboard test program sooner I may have used that to test the rest of the key presses. The one I found though gave rather less information then the “Retropie setup routine”. As it was not able to distinguish between “Left Alt” and “Right Alt” etc.