When you rewind an early Gibson humbucking bobbin, you will see various numbers inside each one. Both adjustable and non-adjustable bobbins have a mold number to identify parts if they become irregular and it’s easy to determine what mold cavity needs repair. On Gibson “T” top bobbins the numbers can be seen on the top of the bobbin instead of the inside wall on earlier bobbins. Cosmetically the pickup didn’t look as nice as the Patent Applied For bobbins but since the pickup came with a cover it didn’t matter.