I think it is all done on the transformer. <snip>

No, not all at the transformer. From the evidence of the machines I have, it depends on the age of the machine. Earlier versions are jumpered on the transformer, later versions have the jumpering moved to the plugs.  Examine the power connector between the cabinet and head - later versions have a six pin plug and the wiring is as in the link Marty posted. I have this on Firepower, Gorgar and Timewarp.  Earlier versions have a three pin connector, with just two wires used, between the cabinet and the head and in this case the jumpering is done on the transformer.  I have this on Flash and World Cup.

In both cases what you are doing is putting two primary windings in series for the 240V wiring and for the 110V these winding are put in parallel.

If you can't follow hat's going on then get someone who does understand to assist.