On the second version of the humbucker bobbin, I always thought there was some special thought behind the embossed “T”. The second version has two sharp inside walls and rounded outside walls and the bobbin could be easily be wound top left or top right on a coil machine. When I asked Seth Lover who invented the humbucker what the “T” stood for he laughed and said, “it stands for the “Top” of the bobbin”. This would keep the winder from putting the bobbin on machine backwards and winding the coil in the wrong direction. If a bobbin was wound wrong and assembled with another, the pickup would be out of phase with itself or in combination with others.