This would most likely be a person who keeps a history of a frontier. You have to remember that as the European immigrants moved west across the North American continent, the frontier (the edge of settlements and civilization) moved slowly west, so that 'frontier' could be anywhere west of the original thirteen states, depending on the year. It could be a person who was on the frontier when it was being settled and kept a record of what happened. It could also be a person at a later time, after the frontier became civilized, that researched early papers written at an earlier time, to establish a history of what happened then.