Walter van Waard found a keyboard made by PROTON, a Dutch company from the 80ties that produced many 6502 and Z80 based computers and peripherals.

It looks like an ASCII keyboard with more function keys (F1-F8) than usual.
Production date is probably not before 1984 since the ICs have date codes from 1983.
No documentation or description is known.

On the PCB you see:

1. The MC6802, an 6800 with 128bytes RAM and an internal clock oscillator
2. The 74LS138 glue logic IC
3. A 74LS14 with a potmeter and capacitor
4. A MC145028 dual flip flop with some resistors and capacitors, perhaps a clock oscillator
5. 2x MC6821 PIAs to sense the keyboard matrix and presumably serial I/O via bit-banging.
6. A K1/K2 connector, where three wires are connected. One wire goes to a pin of the 6821 PIA, the other two are power supply 5V and ground.
   When a key is pressed, the 6821 pin goes to 0V, so it looks like a (poor mens) rs232 signal.
7. A keyboard matrix with many extra keys (F1-F8, cursor keys, CHAR Insert/Delete, numerical keyboard.