Hey Tom
I just realized you are talking 2426's. The screw on adaptor plate is aluminum. You are not tapping the field directly but what ever stray magnetism is getting through the spacer and the airgap around the ferrite ring. You have at least a 1 inch gap there. The ferrite ring will start to "grab" the plate that close. I don't think you really have too much to worry about. If they can shield drivers with steel cans and bucking magnets I would think this is more benign than either. Is the spacer bar magnetized from a previous driver???
Rob