Very interesting, I wish I was that young the first panel I fitted was Rely a Bell panel in 1970 the actual panel was wired with anti tamper wire, all windows and door's were wired the 'old way' it was really hard work the back up battery was an Ever Ready type battery, the alarm was connected to a TYPE of dialling machine that had a specially made record placed on a turrntable which sent a message to the Police, there was also another type of Police calling device that went direct to the local 24 hour manned Police Station where we had access to test, at the Police Station there was banks of switches that activated when signalled the switch relay went down an an audible signal was heard the Police Officer then checked the numbered switch and dispatched a Police car.
Believe me this is true even I often wonder if all that really did happen after what we have now days.
i'm not sure your memory serves you to well mate, especially about the 12 volt battery. rely-a-bell was 5 qs flags in the panel and 3 x 'O' cells end of line. Burgots had an EverReady HP1 as standby as it was a 12 volt system.
the PIU (Police Indicator Unit) was fitted to the local police station. a high security 'balanced' version came out later which had 8 channels and had to be 'tuned' both ends - nightmare for service in electrical storms if lightning bolt hit the manhole outside the main police station - which it did 3 times over one easter weekend at Southend - and i was on call.
the '999' you speak of which were 12 volt battery operated was called an 'RG' or 'RE' , one version had line fault sensing, a good clump on the catch side of the case used to send the arm across the record and cancel the call
servicing was a bit 'brutal' in those days B) .
I will not say who I am for security reason's, (I changed to a Govt Dept in the 70's)but I believe we do know each other especially as you worked in Southend nick, the banks of about 10 switches in each bank they were were behind the front office had 3 set positions up middle or down as you say, they were a pain in the Harris I must admit it was a long time ago and my memory is not 100% tend to be clear about good things to remember but bad things it is easier to forget.
Do you remeber Tom Buckley