One important factor in this thread is in my mind, the fact that a tamper loop on the door contact provides double pole protection to the device, if an attempt is made to short out the wiring to prevent the contact from working, a fault condition will be generated. A single pole pair to the device will just stop working if the wiring is shorted out.
As for old alarms, I can go back to the days of end of line batteries, relays, circuit wire, foil, tube and wire frames, infra red rays, and inertia's.....it is much easier nowadays with programmable control units !!! Feeling rather old now :whatchutalkingabout