220v wiring question

[mason]

Looking at the 220v outlets I have they are all 2 hot wires and a neutral wire with the ground attached to the junction box. Looking at a schematic of a welder it shows the 3rd blade on the plug as a ground, can someone explain this to me and how would I change my outlet from a 10-50R to a 6-50R?

I'm confused on how to wire the outlet to work with a welder. Just to clarify I have a red, black, white and copper uninsulated wire.

Thank You