To put it more simply, most machine models have their specific case, while the bobbin is usually of a more general standard, like class 15 or 66. I don't think very early Singer model 15 had the little finger pointing either 11 or 1 o'clock. There were some develoments in the bobbin and hook area in the early years. Neither model 99, 66 or 201 have the same bobbin case, all take class 66 bobbins.