The unmarked card is the interrupter card that provides lamp flash and interrupted ringing. That must be installed in the uppermost slot below the power supply. The 601 didn't include an intercom card, in fact it was a very expensive option.
The 401B is the ONLY intercom card that you have. It's not a dial-selective intercom unit. It just provides a talk path and relies upon manual button/buzzer signaling between stations.
The 403A is definitely the music on hold interface card and there is a dedicated slot near the bottom for that. It is the second one from the bottom.
