This BRAND NEW 1" Diameter "M" Style Bobbin Case has part number BC-DBM(1) NBL
These are Bobbin Cases with NO BACK LASH spring inside.
Anti-spin spring NBL is built into the bobbin case assembly to prevent the bobbin from spinning when the machine is suddenly stopped. If the bobbin spins when the machine stops, there is a good chance that a single loop of thread will fall out of the bobbin case. This condition will cause the thread to snap as soon as the operator begins to sew.
No Back Lash Bobbin Cases are also required by sewing machine manufacturers for use in machines with bobbin thread undertrimmers. The anti-spin spring adds slight additional tension to the bobbin thread and stabilizes the bobbin thread just prior to actuation of the trimmer mechanism.