Trying to understand an old button matrix

I’ve got an old Mephisto europa A chess machine that I want to hack to make an arduino synthetizer.
I want to be able to get an input from the button matrix. It has 8 pins and another cable which seems to be connected to it.
I’ve tested hooking up a multimeter and pressing all buttons, but I got no contact anywhere on the pins. I’m wondering if the black cable could come in handy for this and how.
Any ideas?

Post pictures.

Note that diodes are common in keyboards/keypads, so the multimeter might work in one direction only.

If it has an encoder built into it, it will need power for the encoder. If you have disassembled it, pictures of the PCB might help.

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The thing has leds on the sides, not in itself. It’s also totally in plastic and I can’t find any screws.
As for the number of pins, I said junk: there are actually 16, sorry 'bout that.
For pictures you’ll have to wait a bit, i don’t have my camera right now.
Have a good day