As with many countries, Botswana allows certain organizations related to the government to send appropriately marked mail for free. In Botswana, organizations that are allowed to use this service include government departments, hospitals and medical facilities, schools, tribal administration, the courts, and official functions of the military.
This site provides information on a typology I have developed for Botswana official free marks, a listing of the marks that I know using the typology I have developed and copies of the original listing from the Runner Post by Dennis Firth and others, and the philatelic exhibit I have created about these marks.