Over the course of 60 years I have had a few different sewing machines. Only my singer had the metal bobbin, the rest have been plastic-and I have kept them all. Is there a way to tell the difference in bobbins,of course, besides the width? My Brothers seems to want only Brothers bobbins. So far I have been successful in keeping them apart from the rest but besides marking them, is there a way to tell which ones they are?