Section 6I. 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, to perform, under the direction of the clerk of courts for the county of Plymouth, the duties of clerk pertaining to the equity proceedings of said court, who shall receive from said county, in addition to the salary paid to him as an assistant clerk, a sum equivalent to five per cent of the salary of the clerk.