Section 6O. The clerk of the courts for the county of Bristol may designate, subject to removal by the court or by the clerk, one of the assistant clerks for said county to perform, under the direction of the clerk, the duties of clerk pertaining to the equity proceedings in said court. The assistant clerk shall receive from the commonwealth a sum equivalent to 5 per cent of the salary of the clerk in addition to the salary paid to him as an assistant clerk.