Each tube is connected to a BIPCO BIP 8806-3 Nixie decoder/driver module.

The stamped label indicates this is a Burroughs Corporation model C 3001-2.

Each module has 14 pins. I haven't had time yet to suss out what each pin is for. I do know that the 8806 has memory and accepts BCD input from DTL and TTL in positive 8-4-2-1 logic.