Written Appointments among the Samurai Class (Kantojo) (官途状)
Kantojo (官途状) is a letter which was used after the Muromachi period which stated, that a vassal who distinguished himself in a battle was allowed by his lord to use a specific government post as his private name. It is also known as kanto no kakidashi or juryo no kakidashi.
In the Muromachi period it became a customary practice among shugo daimyo that their vassals were allowed to call themselves after the official position or title of the military officer or the governor of the province. This was called juryomei. The permission to be called with a juryomei was issued as a letter by the lord to his vassal, which was called kantojo. Kantojo functioned as one of the reward grants, similar to increase in territory. In the first place, one can say it was essentially an invalid practice as it was no more than a private name between a vassal and his shugo daimyo, and was not under control of the Imperial Court. But this customary practice was not controlled as provincial military governors (shugo) were influential and samurai played a central role in society. During and after the Sengoku period, Kantojo was often used as a reward for military exploits.
As might be expected, however, such name could not be used at official places like the Imperial Court or the bakufu (Japanese feudal government), so with the intention to avoid privately using the government post names, a custom was born to use abbreviations of official government posts. It became popular among samurai and resulted in using hyakkan na and Azuma hyakkan.
After the Edo period, the custom remained among samurai families who once lost the position of daimyo during the end of mediaeval period to issue Kantojo mainly to their old retainer who stayed in villages.