tried to change my post but ran out of time  
i typed all of the below and i couldn't post it so i had to type it all again  

what i meant to say is the way the flippers are controlled is different
 wms uses a separate flipper board to control the flippers and the sterns uses the driver board to control the flippers.

wms has 2 windings, one is a power winding which brings the flipper up and the one is the hold winding which holds the flipper up. (they use a transistor for each winding)

stern uses a single flipper coil winding, once the flipper is up the flipper coil is turned off and on very quickly (pulsed) to hold it up. (they use one mosfet for one winding)
this is why even new sterns have some flipper hum when you hold the flipper button in.