The 6v will almost certainly refer to the coil voltage i.e. the amount of electrical grunt you need coming from the receiver supply to tug the relay contacts across. As mentioned on another recent thread, most relays will operate at about half their nominal coil rating, so the standard 4 x AA Rx pack will do nicely. Take care you check the order of the wires in the plug - from memory, Derek Olley decided to make his with the negative lead in the centre, so you may well need to change them over if you're not using Fleet gear. Most contemporary gear has centre-positive.
While I can't find any specific info on this elderly beast I reckon you'd be OK to use it on up to a 12v motor battery, and maybe 15A as an absolute maximum continuous current. Fuse it @ 10A just to be safe, and good luck!
FLJ