Different plug on cord

I have a valve seat grinding tool and never pay much attn to it other than that it had a 2 prong non polar plug on it,, cord was frayed so I finally say this is the day and work up the motivation before I grind up a head to change the cord and its a steel case tool so I am going to put a new piece of sjow and a 3 prong plug on. Take it apart at the switch and see 3 wire cord. I check to see if someone change the end but it looks factory and I fig this is old,,, obviously before polarization and grounding. I look closer and I notice a teet on the side of the plug. This must have been their first attempt at providing grounding means, it must have had green wire coming out the side of the thing and was intended to be hooked up to the screw in center of a recept,, someone had clipped it off. I tape up a spot in cord and change the plug,, but thats a first for me,, had never seen that before and have run into some old stuff,,,, Just wondering if some old timers have seen those before?

 

My dad had several power tools in the early 60's with that arrangement. I can't say how old they really were.

When I was 12 or so, I used his drill one day before he came home from work. I left it plugged in and when he came home, he gave my butt a good whoopin' before he showed me how to connect the ground wire -- and why.