I have no idea, a mystery indeed  ... I have a few SS coil stops on my desk now and they do not appear to be magnetic at all. Only EM ones are in machines so I can't readily tell.

It might be worth mentioning that there are a few EMs that use DC for coils, so it's not strictly an EM v SS thing, it's an AC v DC thing, but that's a side issue.

What PBR says is ....

"Coil stops for Williams/Bally games for 20 years were different parts for AC and DC coils. They were also a common part that had to be replaced due to wear. These were found in pinball machines from the 60's to the 80's. The Pinball Resource re-engineered a single coil stop (one for Williams and one for Bally's) for both AC and DC coils. The support bracket was strengthened by increasing the metal thickness, a copper sleeve was added, thicker cap, longer shafts, etc... The picture to the left shows the parts you receive from us!

Williams Coil Stop AC/DC WLL-A8143+ $4.50ea

Bally Coil Stop AC/DC BLY-A613-67+ $4.50ea"

I assume the copper sleeve mentioned above is what makes them non magnetic?  When you look at the SS ones it looks like there is a thin non ferrous metal layer, like a washer, under the base of the main stop.