i understand all comments and am not knowledgeable of this type of situation.
I have now taken the following photograph with my camera that does show that the "15" is definitely over the black.
The yellowish colour was created by placing a strong table lamp to light up the stamp while being photographed.
The original image was from my scanner.
Please consider the following additional two points:
The black area is not the same depth of shade as any of the cancellation on any of the image area.
Aligning the straight edge of the cancellation with the straight edge of the black area is not 90 degrees (closer to 95 degrees), which one would expect with a right-angled square or rectangular cancellation machine.