Intial examination:
The logic on the board is fully combinational, no pesky clocks or other noise inducing signals.
The value of the 16bit BCD inputs are latched via IC2 and IC3 (74HC573A Octal TransparentD-TypeLatchesWith 3-State Outputs) and displayed on the front 4 digit 7-segment display, via the decoder ICs 7-9 (HEF4511BT BCD to 7-segment latch decoder drivers).
Also the input values are entered in a BCD to binary converter EPROM IC14 (M27C1024 64KBx16 UV EPROM), the outputs go into IC15 (AD7535JN 14bit DAC with parallel inputs ) that uses as reference IC16 (AD587KJ or JN 10volts precision reference).
On the board are a large number of precision OP, either OP07 or OP177, some relays and a mystery EPROM, IC1 (M27C128 16KBx8).
On the front panel there are two 2 positions, non-retaining switch ( Ein - 0 - Ext ein) and (Aus - 0 - Setz), most likely switching the reference with some external one.
Recommended BCD switches:
https://www.ebay.de/itm/Kodierschalter-0-9-BCD-Codierschalter-Vorwahlschalter-Pushweelswitch-1-Stuck/312231336643So much for now, lots of things TODO.