Mexico, Scott 9a, 1861 Second issue, 4r black on yellow Hidalgo Diagonal Bisect on Commercial Wrapper.  Bisect tied to cover by nice strike of SAN LUIS POTOSI cancel (Schatzkes #1443), addressed to Mexico City.  FRANCO handstamp on front, large black "1" postal marking indicates 3 reales postage was paid instead of 2 reales, additional black oval CHABOT HERMANOS - ST LUIS POTOSI handstamp of sender, fresh, VF and rare.  The diagonal bisects are 4 times as scarce as the vertical and horizontal bisects. ex. Guitierrez, Dr. Gene Scott.  Accompanied by a 1994 MEPSI photo Certificate which states:

"1861, 1/2 of 4 reals black on yellow used as 2 reals, (Scott No. 9a), tied to cover by a S.L. Potosi cancel (Schatzkes No. 1443).  The cover also has a "Franco" marking on the front and a "1" on the back indicating that an additional 1 real postage was paid by the sender, it is genuine in all respects.".  

A showpiece.