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

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.