The newer style coil stops have a brass ring/washer between the stop and the plate. Based on other uses of brass washers in EMs, I'd guess this is to break up the magnetism.
http://pbresource.com/rebuildk/gtb-a17908.jpg
From an old, old tip of Tim Arnold's"
"25 - A.C. PARTS - D.C. PARTS
Magnet coils on EM games contain a brass or copper cladding that retains the magnetic field thru the "0 cycle" of the A.C. wave! If it comes loose or wears out, relays will start to hum! Replace Coil! A phone call to Steve Young or myself can cross reference 95% of all coils! D.C. coils contain no such copper slug, but do have a diode across the winding! As the magnetic field collapses, a "spike" of reverse voltage comes out of the coil! The diode suppresses this! Don't ignore this! On most relays there is a brass or copper washer between the coil and the relay frame! This, along with the brass screw keep the frame isolated magnetically! IMPORTANT! As relays on Gottlieb games need ONLY to have coil removed, cleaned, de-magnetized, and strike plate wear spot covered with mylar. DO NOT attempt to take stepping part of relay apart!"
The older style of coil stop and plate don't have this brass washer wedged in.