Susie's right about that. We keep extras in our bobbin box.
Oh, FYI, there are at least two variations of the Class 15 bobbin cases for the Japanese machines, possibly more.
Short needle slot: If the machine is a vertical end loading ZZ the bobbin case will actually move left and right with the needle. It takes a case with a short needle slot.
Long needle slot: If the machine is a vertical front loading ZZ the bobbin case is stationary and the needle needs a wider slot so it does not hit the case.
Straight stitch machines use the short slot case as OEM, but can actually use any of them.
Joe