Paraguay, Scott C175 variety, MNH.  1948 10g dark green & brown Air Mail with INVERTED CENTER, choice sheet corner example, fresh, bright & XF.  Unlisted, rare. 

Accompanied by a 2007 APEX Certificate which states:

"Paraguay, Scott No. C175 with center inverted (unlisted), unused, full original gum, never hinged, genuine in all respects."

25 exist, 5 of which are in se-tenant pairs with a normal stamp.