Each employee under section 5-268d of the 1969 supplement to the general statutes shall be credited for retirement purposes under this chapter with his period of full-time service as an employee of any such bookstore upon payment into the State Employees Retirement Fund of such contributions as he would have paid if he had been a state employee during the period of such service and his salary for such service had been paid by the state, with five per cent interest on such contributions from July 1, 1968, to the date of payment.