Is there another row of connectors under the cable? For 83 switches you need at least 19 pins (10x9 matrix would allow for 90) and there are only 10 visible. When you know how many there are you can compare to the number available on the Pro Micro.
Basically you need to work out which switches are connected to each other, but it's a one sided PCB using switches with two pairs of pins which are used to bridge over connections. I tried to map one group and it seems some of the switches are mounted sideways - I have so much respect for the guy who designed these boards, it shouldn't be possible
) I've attached my attempt but not confident due to possible switch rotations and apparently redundant connections, even if it is right there are more connected parts I'm sure...
If you don't like the 'colouring in' option you can map out the connections however your mind works - I've used a multimeter to test the connections and noted them in a spreadsheet with two columns for each switch which which worked out OK.
Any questions please ask, I like saving old boards so it's no problem at all.
