Bobbin antibacklash springs are not interchangeable! Here’s how to tell which you need:
Hold the case like a letter C, with the opening on the right. Tilt it and look at the slot position. If it is a smaller slot closer to the finger that holds the bobbin in place when you hold out the hinged “door,” you need the football style antibacklash spring, shown above in the case on the left. If the slot is wide and the distance greater, like shown in the first photo on the second case, you’ll need a circular spring, also known as an older style spring shown in the second photo on the right.