They were printed in sheets of 400, then severed(cut) into 4 panes 100. So you have an inside edge of the Left 2 panes, if you had the straight edge on the left hand side of stamp, it would have been from the inside edge of the 2 right hand panes.
similar to the Admiral series
http://www.bnaps.org/ore/VanSomeren...ightEdge.htmTrodent