I totally understood that CD.
The reason I asked is that my 15-91 is missing the bobbin and the case. I have several for other machines so I found one that looked like what I needed and stuck it in there. It fits but the machine seemed to bind when turned by hand without the needle. I also have several styles of class 15 bobbins but I haven't tried any of those just yet. I need to make sure I have the right case before moving on to the bobbin part. The machine turns freely but with a slight drag due to the motor without the bobbin case in place.