In late July 2012, Iridium 51 (25262, 1998-018A). which had been out of the operational constellation for many years, was moved to the postion previously occupied by Iridium 7 (24793, 1997-020B), while Iridum 7 was moved to follow slightly behind it. The two satellites are each providing some of the functionality for the given slot. See a copy of the related FCC filing. Orbital Plane 4 has no other spare satellite.