A member may purchase service credit as provided in subsection (b) of this section for periods of employment described in subsection (c) or (d) of this section for which the member is not otherwise entitled to service credit. A member who purchases service credit under this section shall: complete a claim for the service credit and file it with the Board of Trustees on a form that the Board of Trustees provides; and pay to the Board of Trustees in a single payment the member contributions the member would have made for the period of employment for which service credit is being purchased plus regular interest to the date of payment. A member may pay for service credit purchased under this section at any time before retirement. A member of the Employees’ Retirement System may purchase service credit for a period of employment: by the State or a participating governmental unit; as a member of the State Police Retirement System for which the member was not vested in that State system; when the member was on an employer-approved leave of absence not exceeding 2 years that has qualified for purchase under regulations adopted by the Board of Trustees; by the Department of Legislative Services, the Office of the Attorney General, or as secretary to the Speaker of the House of Delegates or as secretary to the President of the Senate during a session of the General Assembly; by a member of the Senate or House of Delegates or the Office of the Secretary of the Senate or the Chief Clerk of the House of Delegates; as a member of the Employees’ Retirement System or the Teachers’ Retirement System for which the member had withdrawn the member’s accumulated contributions after a prior termination of membership; or when membership was mandatory but for which member contributions were not made. For service credit purchased under: paragraph (1)(iv) of this subsection, a year or part of a year of employment equals 1 year of service credit; and paragraph (1)(v) of this subsection, employment for at least 130 days per year as a legislative employee equals 1 year of service credit. A member of the Teachers’ Retirement System may purchase service credit for periods of employment: during which a member held an emergency teaching certificate; as a member of the State Police Retirement System for which the member was not vested in that State system; when membership was mandatory but for which member contributions were not made; when the member was on an employer-approved leave of absence not exceeding 2 years that has qualified for purchase under regulations adopted by the Board of Trustees; as a member of the Employees’ Retirement System or the Teachers’ Retirement System for which the member had withdrawn the member’s accumulated contributions after a prior termination of membership; or by the State or a participating governmental unit on a permanent basis.