There are lots of different class M bobbins. I went through the same thought process you are going through and ordered some on eBay. They weren't the same as mine, but the seller was very helpful and allowed me to return those and sent me detailed pictures showing the others that he had in stock. One of them fit and sewed well enough, but I have since decided to stick with the manufacturer's bobbin cases. As you point out, they are expensive, but they are machined more precisely.