Doing this from memory as an ex RR Rep...
I believe the designations refer to the fire detection system installed. There were two possible options, a pneumatic type detector manufactured by Systron Donner whereby a gas would expand in response to temperature increase closing a switch to alert the flight crew of an overheat/fire condition. The alternative method was to monitor for a change in electrical resistance as manufactured by Graviner.
Having the two systems was a PIA because it was yet another configuration detail to be catered for during engine change especially with lease engines that may have a different fire detection system to that being removed. Subsequent RR engines only came with one fire detection action system.