If one bought a used machine, how would one know which type to use?
My machines are all older, and the instruction books (for some of the machines, anyway) didn't even bother to mention what type of bobbin(such as 15, 66, whatever) to use - much less whether plastic or metal was more appropriate. As far as that goes, some of the machines may be "pre-plastic bobbins"
I sure wish bobbins had some sort of imprint on them stating what they are.