Section 6J. The justices of the superior court may designate, for a term of three years from the date of such designation, one of the assistant clerks for the county of Plymouth, appointed under section five, to perform, under the direction of the clerk of the courts for the county of Plymouth, the duties of clerk pertaining to the criminal proceedings in said court, who shall receive from said county, in addition to the salary paid to him as an assistant clerk under section five, a sum equivalent to ten per cent of the salary of the clerk.