The stamp … must come from plate 13 or the upper half of plate 14.

As Marler points out, most subjects on the proof of plates 13 and 14 have a nick in the UR corner of the frame. However, many stamps were retouched after the proof was made and the nick removed as a result of retouching. But, interestingly, there is a nick on the stamp with the scratches. Perhaps what happened is that the scratches occurred during the extensive retouching of plates 13-14 even though the stamp with the scratches was not retouched.