You need to find out what detection there is on the system, at that age I wouldn't be surprised if it's a detector, a door contact and probably a (disconnected) pressure matt.
Once you know this you can systematically work our what detection is in part set v full set.
There may be a fused spur nearby, alternatively it can be done inside the panel but it hurt's if you get it wrong so personally I recommend you get someone over to check it out or even down-power it for you.