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| A very interesting sello negro usage of a letter mailed from Piedras Negras to Monterrey on February 27, 1863. Apparently the cancel had been misplaced as the letter bears a manuscript "Franco" and "Piedras Negras" and date. |
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| A letter mailed from San Antonio, Texas where it was carried to the border town Piedras Negras. There it was placed into Mexican mail for delivery to Monterrey. Mailed from San Antonio on September 30, 1869. |
