Any permanent policeman who is a member of the state employees’ retirement system on July 1, 1957, and who is eligible for membership in the policemen’s retirement system, hereinbefore set forth, may elect within 3 months after said date to transfer his membership from the state employees’ retirement system to the policemen’s retirement system in accordance with the provisions of N.H. Rev. Stat. § 100:21 to 27, relative to transfers of membership between state retirement systems, anything in said subdivision to the contrary notwithstanding.