In August 2005, Iridium 17 (24870, 1997-034B) evidently failed, and Iridium 77 (25471, 1998-051E) took its place in the operational constellation.
This was first highlighted by Bjorn Gimle as Iridium 17? on Tuesday, August 30, 2005 on SeeSat-L -see http://www.satobs.org/seesat/Aug-2005/0332.html and http://www.satobs.org/seesat/Aug-2005/0335.html.
This now left orbital plane 6 without a spare satellite.
Whether an attempt will be made to move a spare satellite from another orbital plane, in readiness for any future failure in orbital plane 6, remains to to be seen.