Hall-effect keyboard query / was Re: Sources for 8b TTL keyboards (Keytronics)

[Brent Hilpert]

With all this talk of old keyboards, I figured it was time to pull out one that
I've had kicking around for awhile but hadn't assessed as yet. It's a
very-ASCII keyboard (control codes are labeled on the keys) using Hall-effect keyswitches:
  MICRO SWITCH
  Freeport, Illinois
  61SW12-2
  SW-10405
  date-stamp: 75/49 (1975)
(Would the MICRO SWITCH indicate this was from Honeywell?)

It appears there are two outputs from each keyswitch which go through a
connection matrix to feed a 28-pin LSI encoder IC:
  AMI 
  SW20416
  7423N
The IC would appear to take a 2-of-12 input code and convert it to ASCII output
(actually 9 output lines not including the strobe, I can figure them out
later). 0V and +5V are identifiable from other circuitry on the board (two 946
DTL ICs), but there is one other pin on the LSI IC which presumably is a MOS
supply voltage.

The question is: does this keyboard sound familiar to anybody and would someone know
what the 'other' voltage for the keyboard/encoder IC is. Connection to the
keyboard is
via a 24-pin PCB edge connector, pin-A is 0V, pin-1 is +5V, pin-B is the unknown
(supply) to the IC. It's likely -12V or -15V, and it's a very obscure question but
I just thought I'd ask before powering it up. 

Very nice, solid-feeling basic ASCII keyboard, stainless-steel metalwork,
double-injected keytops, in good condition, just missing one of the SHIFT keytops.
Presumably came out of some ASCII terminal.