lowjane, You have me quite confused. What I am hearing is that you have the foot pedal plugged into a 3 prong receptacle and it works. There is a dual receptacle on the same machine??? That would be a rare beastie indeed.
I am guessing that someone may have just put a 3 wire machine into a far older case/cabinet that had the dual plug in and disconnected the dual plug. The usual swap fro a knee or a foot is just to use the 3 prong. Pics perhaps??