Ok I dug around and found a handful of different class 15 bobbins. They are about the same size around a a nickle - at least that nickle. They are all about the same depth they should be flat. They need to be straight and round, free of rust, old thread, burrs and what not. The newer bobbins may not have the same size hole to go on the bobbin winder shaft. Some of the newer bobbins are not as strong. The plastic new bobbins are better than the metal new bobbins. I used to have one made out of aluminum but I can't find it any more. When they are in the bobbin case they should be flush with the edge.
Old new stock - new stock and old from the box
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