(a) Except as provided in subsection (b) of this section, in computing the length of service for municipal retirement in accordance with the provisions of § 7-436, no period of employment during which a member was eligible for membership in any pension system established by or under the authority of any special act shall be counted as service with respect to the computation of his retirement allowance.

(b) A member may receive retirement credit for a period of employment described in subsection (a) of this section if (1) such member is not receiving or entitled to receive a retirement benefit based on such employment, (2) such employment was in a municipality or department for which participation under this part has subsequently been accepted and (3) such member contributes an amount determined by the Retirement Commission to be necessary to fund any increase in benefits resulting from receipt of such credit to the Municipal Employees’ Retirement Fund.