Any member retired on or after January 1, 1975 who is an employee of a county department of social services who was merged and placed under the employment of the state department of social services pursuant to Act No. 280 of the Public Acts of 1939, as amended, being sections 400.1 to 400.122 of the Michigan Compiled Laws, shall receive retirement system service credit for the period of service under the county department of social services if the employee withdraws all rights in and to any pension benefit he may be eligible to receive under a county pension plan, and pays into the state employees' retirement system a contribution of $600.00 for each year of service claimed.