Nice find. GT4 tagging is the standard on this issue in the booklet but the stamp is centered high, creating a three bar tag variety on the stamp itself. Congratulations, you have just discovered a new variety that I believe would be would be a G4aB.
Edit: I may stand to be corrected, as the top of the stamp does not contain a white border or the image "breaks out" . And lately the issues where there is "breakout of image" or no white border, it contains no tagging.