On 15 May 1864 the regency replaced the existing stamps with the Eagle stamps depicting the Coat of arms of Mexico, an eagle killing a snake. A 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.