there are only the two wires for the motor, the earth mounting point has NO wire!!!
My sieg lathe used to give little electric shocks. it came with a funny looking plug and ran through an adapter, i changed the plug and found NO earth!!!
EEK!! Doesnt sound good. Have you a multimeter to check (with the lead unplugged to save any

) if either of the wires going to the motor is connected to any of the pins in the mains plug? Assuming that neither of the 2 wires are connected to the mains (either live or neutral) or any metal casing, then you should be safe to reverse the wires to make the motor run in reverse.
Might be a good idea to earth the metal parts of the mill, and make a wired earth connection to both the mill frame and the mill head (dont rely on a good electrical path from just the metal to metal contact of the mill head to its column. If in any doubt, consult with a local electrician.
Tim