The high school I attended (going back to 1993 here) had an old Executone intercom system - the thing was huge by today’s standards, about the size of three washing machines, but probably “state of the art” in 1969 when it was installed. But anyway, I remember there was three switches near the top for “emergency overide”, one of the switches was labeled “air-raid”. Never knew what they actually did but assumed they broadcast some sort of tone over the speakers for whatever event was going on. Your fire alarm system must be tied in to some auxiliary function of the intercom system. Question is when was this done? Could be possible is was an approved method back in the day, even though it would not meet today’s standards. It is quite possible too the time clock system is an independent component from the intercom, and is simply connected to an auxiliary function of the intercom system, just like the fire alarm. I would imagine there is a priority to the system and the time clock got in front of the fire alarm on the priority and was never actually checked.