crown was added to the eagle to indicate the monarchy and the spelling was changed from "Mejico" to "Mexico." The stamps were engraved and issued imperforate. Five values were initially issued; 1/2r, 1r, 2r, 4r and 8r, to which a 3c. was added in 1865. The stamps were shipped from Mexico City to the district offices where the district name usually would be added to validate the stamps. Starting in July 1864, the stamps were overprinted with the year and an invoice number prior to shipment to the districts. The various overprints were made in different sizes and based on such variations, specialists have distinguished five separate issues, referred to as "1st" through "5th" period eagles. This issue is very popular with collectors and detailed studies have been made of the issue, including information obtained from surviving postal records. Another study of the Eagle stamps is on Jesper Andersen's site. Maximilian issue 1866 On 1 August 1866 the regency issued a new set of stamps depicting a profile bust of Emperor Maximilian. This issue was lithographed, rather poorly, and issued imperforate. The regency had previously changed the official currency under which one peso equaled 8 reales to a decimal system in which one peso equaled 100 centavos. In making the conversion postal rates were rounded up, for example, the rate of one real being equal to 12.5 centavos was rounded up to 13 centavos. The stamps, as a result, were issued in values of 7c, 13c, 25c, and 50c. These stamps were issued with the year and invoice number and with or without the district name, but some issued in exchange for Eagle stamps were issued only with the Mexico City district name. On 16 October 1866, the lithographed Maximilians were replaced with a finely engraved version of the same design and values. These were issued imperforate and with year and invoice number, with or without district name. Stamps without overprint are unissued remainders. Sellos negro Maximilian's Empire was never in control of the entire country and was in constant battle with Republican forces led by the ousted president Benito Juárez. Juárez eventually set up a government in exile in Chihuahua. Parts of the country not in control of the regency would not use stamps bearing Maximilian's image and issued a few provisional stamps. More commonly those areas reverted to using pre-stamp postal markings with "franco" stamps. These stampless covers