Campeche, un real.
Third period, consignment 222-1864.
Image courtesy Manuel Iglesias.
Note the district name overprint: it was applied after the stamp was affixed to the letter, tying the stamp to the letter. This may be the reason why we rarely see the district name on Campeche stamps - it may have been imprinted entirely on the letter, away from the stamp. A similar occurence happened in Soyaniquilpan and Ixtlahuaca, for example.
Eagle stamps from Campeche are rarely in good condition. The image above shows an unusually nice copy.