The 600 series use a bunch of different plugs and I'm pretty sure some of them have an "extra" hole in the plug that doesn't have a matching prong on the machine. I don't have that exact model but do have a machine plug has an extra hole that doesn't have a corresponding prong on the machine and works fine. I'm glad Rodney has that model to double check. I think that if you look closely at the holes in the plug that the hole that doesn't have a matching prong is shaped a tad different than the other three.
Janey - Neat people never make the exciting discoveries I do.