Member of first owner's family says it was used for a car tracking/security service. My earlier Omega (This/current one is 2002) had 'Tracker' fitted but with no visible aerial.
Probably used on the Securicor Datatrak Service then. Securicor developed their own AVL Service in the UK for their own Security Vans when early GPS System was deemed patchy or unreliable for non Military use. The Vans would pick up HF Beacons around the Country, calculated its position then transmit the Data back to Securicor Control Rooms via its own UHF Radio Network.
Having built its owm AVL Service, Securicor Datatrak would also Sub Rent its Services to 3rd Party Firms for increased revenue.
The System you had fitted, Tracker, was slightly different design, in as much a Tracker (AVL with Open Transmitter Data) would be hidden in the Vehicle, then activated by forced entry or remotely, transmit its location in real time on an open common frequencies so that the Police, Recovery or the Tracker Control Rooms could receive and Track the Data from the Vehicle.