In late July 2012, Iridium 51 (25262, 1998-018A). which had been out of the operational constellation for many years, was moved to the position previously occupied by Iridium 7 (24793, 1997-020B), while Iridum 7 was moved to follow slightly behind it. The two satellites were each providing some of the functionality for the given slot. See a copy of the related FCC filing. Orbital Plane 4 had no other spare satellite.
In October 2014, Iridium 51 was moved around the orbital plane so that is was paired with Iridium 6 (24794, 1997-020C)
In June 2017, Iridium 51 was moved back around the orbital plane so that is was once again paired with Iridium 7 (24793, 1997-020B)