Jim & Phonenut are correct. The 401 A required an 'A' lead; the 401B did not. They both provideded the same thing - a filtered talk path. You would use them with a button & buzzer system or with a matching transformer (WMT-1) as an access to a PA system (there was a contact closure that worked really well for muting the BGM.

Sam

"Where are we going and why are we in this hand basket?"