The Custos Brevium was an official in the Court of Common Pleas and Court of King's Bench. In the Court of Common Pleas the Custos Brevium served as Chief Clerk, in charge of the officials that supported the Justices of the Common Pleas in their business. In practice the position was a royal favour, and the actual clerking was done by the Custos Brevium's Deputy.
The Custos Brevium of the King's Bench is a much more obscure figure because he was not appointed by the King. The office of Custos Brevium of the King's Bench was combined with the Clerk of the Treasury and Clerk of the Warrants by the 17th century, and there is enough evidence to suggest this had probably occurred by the middle of the 15th century.
Read more about Custos Brevium: List of Custos Brevium of The Court of Common Pleas, List of Custos Brevium of The Court of King/Queen's Bench