CENTRAL CIVIL SERVICES (REVISED PAY ) RULES, 2008 -THE RE-EXERCISE OF OPTION UNDER RULE 6 OF THE CENTRAL SERVICES (REVISED PAY) RULES, 2008 IN CASE OF EMPLOYEES COVERED UNDER THE O.M. DATED 19.3.2012. 
http://www.indiapost.gov.in/DOP/Pdf/Circulars/25-10-2005-FS_03-01-2013.pdf