Thanks to a nice posting about resetting the neck on a similar guitar by Jim Fleeting I learned that some Gretsch guitars have a screw through the heel of the neck which is coverd by filler and finished over. Your can read his posting here. In fact, the outline of the hole is visible in the right light. A rare earth magnet can also be used to detect the screw below the surface. I dilled out the majority of the filler and carved away the rest with a small chisel. The screw backed out without any problems.