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If the circular post is equidistant from the 4 spade connectors, then it
might be some kind of a voltage selector. For example, connecting the
circular post to the top left might make the unit work at 120V, versus
connecting it to the top right might make it 240V. Just a guess, please don't
sue me if you get fried and all your hair falls out.
Comment: Most places that repair electrical equipment have a variac
transformer. This is a device that can put out a variable voltage anywhere
between nothing and the supply voltage. When I salvage dumpster equipment, I
like to connect it to a variac and increase power slowly, just in case of a
bad outcome. (Any outcome that includes fire or electrical shock hazards is
considered a bad outcome.)