(a) A member may purchase service credit as provided in subsection (b) of this section for periods of employment for which the member:

(1) (i) would have been entitled to purchase service credit under the Employees’ Retirement System; and

(ii) is not otherwise entitled to service credit under the Correctional Officers’ Retirement System; or

(2) was not vested in the Employees’ Pension System.

(b) (1) A member who purchases service credit under this section shall:

(i) 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

(ii) 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.

(2) A member may pay for service credit purchased under this section at any time before retirement.