Fonas Tri-1 has an unlabeled 40-pin DIP that I haven't been able to figure out. The PCB has a date code in 1979, and one trace is marked VSS 0 and another trace is marked VDD -15, so it's NMOS. I desoldered it and there are no markings on the bottom or on the PCB under it. I haven't had any luck with patent searches. Any ideas? Here's the pinout:

1 N/C - looks like clock output - I measure 86.5 KHz at 12.5% duty cycle
2 VSS
3 VSS
4 resistor to VDD
5 VDD
6 VSS
7 VSS
8 switch input
9 button input
10 switch input
11 button input
12 button input
13 VSS
14 button input
15 VDD
16 VSS
17 reset button - not sure if this is an actual MCU reset
18 out to LED
19 out to LED
20 out to LED
21 out to LED
22 out to LED
23 out to LED
24 out to LED
25 out to LED
26 VDD
27 N/C
28 VSS
29 VSS
30 VSS
31 out to 75492
32 out to 75492
33 out to 75492
34 out to 75492
35 out to 75492
36 out to 75492
37 out to transistor
38 out to transistor
39 out to transistor
40 out to piezo