This is a kind of cool old board with 3rd gen Fujitsu leaf spring switches.
Opened:
Desoldered the ribbon cable attaching the top PCB and traced out the matrix to wire the Teensy.
BTW this PCB seemed a little touchier than most, I did lift a pad and so used a bridge wire. Can also see part of pad on pin 3 got lifted too!
Then wired the LEDs and speaker to the Teensy, reattached the the top PCB and closed it up. I did set up a key to toggle the speaker so I don't have to listen to it all the time.
Very nice smooth switches IMO. Here's a short video of typing on it (well banging on some keys anyway):
The matrix I came up with if anyone is interested: