OK, I was able to locate some additional info.
It is a Civil War Propaganda stamp issued for Lugo. It was issued in panes of 50 (5x10). In the full pane, the 6th stamp in the 3rd row is inverted. I don't know if that happened for all the printing, or only a few panes.
But my guess is that the inverted stamp was likely deliberate (my opinion). Here is a pilfered small pic: