Turning over one of the bobbins on a humbucker will make the pickup out of phase with itself. You are electrically changing the direction of the coil and making it incompatible with the system. Depending on what pickup you are using will not look very attractive from the bottom side. The top of the bobbin is made to look attractive and the bottom of the bobbin has less attention and the mold is not polished. Often the lead wires exit from the bottom of the pickup and often the bobbin has many tooling and ejection marks. During the mid ’60’s Gibson made a new bobbin often called the “T” top. The “T” was put on to designate the “top” of the bobbin. This identification would insure proper mounting when winding and help eliminate pickups from being out of phase when the two coils are hooked up together.