Notwithstanding the provisions of section 5-175b, a member of the state employees retirement system who has been in the active full-time employment of the United States federal government for some past period or periods for which he has received or will receive no retirement benefit or pension and for which he is not eligible to obtain credit for retirement purposes under any section of this chapter may receive credit for such period or periods, not to exceed ten years in the aggregate, by making contributions to the State Employees Retirement Fund, for each year of such prior service, equal to two, or five per cent of his federal salary, as appropriate for plan membership, for the period of such federal service, plus interest thereon at the rate of five per cent per year from the time such service was rendered to date of payment. Such payment may be made by payroll deductions as determined by the Retirement Commission over a period not to exceed twenty-four months, and such installments shall include interest at five per cent per year. No service credit shall be granted unless payment of all contributions and interest is completed, and then not more than one year of federal service shall be counted for each two years of Connecticut state service.