Over the years there have been design changes due to various reasons. In time, the injection mold can wear out and leave the bobbin out of specifications or excess plastic or flashing on the edges that can cause problems when winding. New designs are created to ease in winding and manufacturing. After the “Patent Applied For” bobbin was discontinued during the mid 60s a new “T” top bobbin was introduced. The recessed square hole was eliminated because a newer procedure was used for hooking up the black and white lead wires to the coil. Various changes often happen when using various suppliers with their many sources. Even though changes may take place I feel Gibson has always kept a high standard of quality.