I started out my career working on FA/BA systems just like that. They also use to have little wheels inside of the "Flow" alarm. The wheel would turn and send a signal to the central station. By the number of clicks you received, you would know which building had the fire. A Flow alarm, is a box mounted to the fire riser pipe. It was mounted just above the check valve. You drilled a hole in the 8"-10" black fire pipe. Curled up a plastic flap ( that looked like a beaver tail) stick the flap inside the pipe to allow it to open up. Then when someone or sprinkler head opened up, the water would flow, push the beaver tail up and activate the alarm. What is even older, (and I wish I can get a picture of it)is the pneumatic fire alarm systems. They used to use 1/4" (or 1/8") copper pipe full of air. When there was a fire, the pressure inside the pipe would heat up and press on a fire alarm button in the main panel.
