The lady at the sewing machine place told me that the repair place calibrates the machine for either a plastic or metal bobbin.
I've used both metal and plastic bobbins with my Pfaff (mid 1980's ) and the old Singers (1960s) and they seem to work okay to me.
I also like to have LOTS of bobbins on hand. Can't waste that last 20 feet of thread on that bobbin, now, can we.