The place for good metal bobbins is listed in Miriam's post about 3 up from your post re plastic bobbins in a metal machine. Not an expert here but this is my theory. The plastic is so light that it unwinds differently (beacuse of lighter weight) than does a metal bobbin. You also find older bobbins wuond with several different colors because people were more frugal and if the spool had some thread left, the ladies or men would put that onto a bobbin vs. just discarding the thread remnant as we are likely to do now. I got about 15 bobbins with one of my vintage ladies and without exception all had at least 2 different colors on them. I hope this helps. ~ Chris
FYI: The metal bobbins don't work n the plastic machines either. Don't ask how I know that.