Lucas made a special tool to undo these screws. I have only ever seen one as it was not an operation we used to do when refurbishing the dynamo. If it needed stripping that bad it was replaced on an exchange basis and the old unit sent back to the manufacturers, so not many workshops had one of these tools.
If you really want to strip it back I would suggest using a pillar drill with a large phillips bit with a rod welded onto the side of it to form a "T". Fit this into the chuck and with the dynamo body fitted into the machine vice line up the screw head with the bit. Exerting a downward force, as if drilling a hole, this locks the phillips bit firmly in the screw head and by using the "T" bar turn the screw out. From memory they are a might tight so this is why constant downward pressure is required to stop the screwdriver bit riding out and damaging the screw head.
Reverse this procedure to tighten the screw when you assemble the unit.