I should also mention that the singer bakelite foot pedal -can't- be wired in reverse.
For example, you will notice that there is no mention of being careful which wire to put to which connector in either of these articles:
http://blog.sew-classic.com/2010/01/...g-machine.aspx
http://blog.sew-classic.com/2009/11/...r-control.aspx
For the same reason as before -
AC current doesn't have constant polarity. All a foot pedal does is break the power going to the motor and add in some sort of resistance to allow variability of speed.