An Act to amend title II of, the Social Security Act to permit any instrumentality of two or more States to obtain social security coverage under its agreement separately for those of its employees who are covered by a retirement system and who desire such coverage, to include Alabama, Georgia, New York, and Tennessee among the States which may obtain social security coverage for policemen and firemen in positions covered by a retirement system on the same basis as other State and local employees, and to extend the period during which State agreements for social security coverage of State and local employees may be made retroactive