post mark

from Bouvier's Law Dictionary, Revised 6th Ed (1856)
POST MARK. A stamp or, mark put on letters in the post office. 
     2. Post marks are evidence of a letter having passed through the post 
office. 2 Camp. 620; 2 B. & P. 316; 15 East, 416; 1 M. & S. 201; 15 Com. R. 
206.
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