Section 6K. 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 Hampden, appointed under section five, to perform under the direction of the clerk of courts for the county of Hampden, the duties of clerk pertaining to the equity proceedings in said court, and such other duties as the clerk shall determine, who shall receive from said county, in addition to the salary paid to him as an assistant clerk under section four, a sum equivalent to five per cent of the salary of the clerk.