I built several, I suppose you could call it three, different keyboard encoders.  Of all of them, this was the last, and also the best working.  This one was designed to interface to a 6522 or 6520 io chip.  This one was capable of encoding up to 64 notes (good for 5 octave keyboards).  I prefered this methode over the diode matrix.  This probably dates from around 1981-82.