Hey Stallzer,
I am not too sure what the deal is on these either, but I can contribute some scans for comparison. Yours is nice because it is imperf, but have a look at these and you can see very similar strikes to yours in among the bunch. I added a photobucket link below the group shot that takes you to that album showing each stamp in further detail if you are so inclined.
http://s996.photobucket.com/albums/...9/Zaragorza/Good luck with your quest, I will be following along as the mystery is uncovered!

Have a good one,
Skilo54