"I mean a jumper wire from the green ground of the receptacle to the
metal box. No other jumper."
That's what any green or bare jumper wire should be.
However, if the metal box is not itself grounded a jumper from the green
ground terminal on the receptacle won't do any good. That's why 2 wire +
ground wire is the only way to go in new wiring. All the older ideas are
inadequate by themselves: grounding thru metal conduit, polarization.
Still, most modern small appliances such as toasters, lamps, and the like
are polarized only, no grounding or double insulation. The cord should be
connected so that the shell of a light socket is connected to the
neutral.
And of course GFIs in the bathroom, kitchen and exterior receptacle
circuits. Not every receptacle, just the first from the breaker in each
circuit.
My trailer has a GFI in the bathroom which feeds the two exterior
receptacles. It should have a GFI in the kitchen convenience circuit but
doesn't.
Al