Mass. Gen. Laws ch. 147, § 13C

In any town which has accepted section thirteen A, or has a reserve police force under authority of special law, and accepts this section, no person shall, after the date of acceptance of this section, be appointed to the reserve police force thereof unless appointments to such force are subject to chapter thirty-one and the name of such person shall have been placed, on or before said date, on an eligible list established under chapter thirty-one and the rules made thereunder for such appointments.