I agree, it was my first thought too, and the most logical conclusion.
It appears to be a right margin stamp with a nice wide margin...but no perfs. Weren't the margin stamps perfed also? Also, there is the fairly large top margin with the beginnings of a frame line for the stamp above (on the upper left), so it can't be a top margin stamp. If perfed so high as to have to have been perfed on the face of the stamp above wouldn't it also show the perfs on the face of this stamp?
I can't say I have ever seen one with such nice margins all around without any perfs. It is a logic defier
