Interface A missing PCB
Uli posted a link in a comment to https://lgauge.com/technic/LEGOInterfaceA/9750(1093).htm which has a link to http://isodomos.com/Lego-Sets/1093.html , that link is dead but archive.org has a copy http://web.archive.org/web/20081121054400/http://isodomos.com/Lego-Sets/1093.html both sites have the pinout, but the latter site also confirms the logic levels as 5V TTL.
I will repeat the pinout here, in case those sites disappear.

Pins 1 and 3: 5V

Other odd numbered pins: Ground

Even numbered pins 6-16: Outputs 0-5

Pins 18 and 20: Inputs 6 and 7


Unlike the interface B which is a somewhat-intelligent device that communicates over RS-232 the interface A seems to just use straight digital IO lines. For bi-directional control of motors the outputs can be used in pairs.
The pinout seems to be an exact match for the user port on a BBC micro. I suspect this is not a coincidence.
It would not be difficult to hook this up to something more modern, a 5V Arduino could probably hook up directly. For a Raspberry Pi you would want something to shift the levels, I'd probably use HCT logic running off 5V for the signals from the Pi to the interface A (this may not be strictly needed) and LVC logic running off 3.3V for the signals from the interface A to the Pi.