If two screws are fitted you must assume two screws are necessary. This is really a mechanical task. I would take apart the meter and try to extricate the piece of copper(or brass) which has the terminal on. Now try to unscrew the seized screw, if a screw driver fails then a pipe wrench on the head might do it, work the screw backwards and forwards with some oil or WD40 on it might do the job. The next escalation is to heat the screw with a gas torch in the hope along with your oil and pipe wrench that you can loosen it.
At this stage its best to abandon the screw, cut the head off and put a dimple in its centre and drill the screw out, it is likely that the pressure of the drill and the rotation will drive the screw further in. this is actually not too bad as the screw is loose now. Stuff a screw driver in the hole and unscrew the screw. The only other thing you could do is to drill out the wire hole, it is likely that the drill will snag and break. Once you have a clear passage, drill another face of the terminal and tap a new thread for a new BRASS screw.
Frank