Yeah and I have seen them under the plate screw on receptacles that still had no equipment ground anywhere in the outlet box. Talk about misleading people that don't know what they are doing in the first place.Yoke serves as the ground point. Remember the little green wire with the spade terminal that was supposed to go under the plate screw?
A situation I see quite a bit of around here is old NM with the ground wrapped around the edge of where the jacket is stripped. Then that part of the cable is fastened underneath the clamps entering a metal box. Then a two-prong receptacle was installed.About the only time an equipment ground is present where a two wire receptacle had been originally installed is where a metal raceway was serving the receptacle installation and was a complete raceway back to the point where the circuit originates.
I like the same yoke theory also. I just can't magine that a production run would leave in the screw. It seems small, but that is real $ in large quantities.I bet they use the same yoke for both receptacles rather than change. Surprised they would put the screw in but it may be cheaper then changing the production.