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'5ths' was one of the titles that appeared on the blackboard the band used during two weeks of rehearsals from september 19th to 30th 2005. At the end of that session, Colin posted a picture of this board on Dead Air Space on september 30th, unfortunately (and probably intentionally) in rather low resolution, so that some of it can't be deciphered:
On march 8th 2006, shortly after sessions with Mark Stent had come to an unfruitful end, another photo of a blackboard (and a perhaps slightly worried Ed) was posted on Dead Air Space. A tour had been scheduled for may and june to escape the frustration of the recent sessions and to breathe new life into the project. Rehearsals for the upcoming dates had begun recently, and this board shows which new songs were at this point considered for inclusion in the setlists. '5ths' was one of these, but would not be played and make no known appearence since (click image for full size):
These three pictures were also posted on Dead Air Space on march 8th 2006 under the headline 'wednesdayfifths'. The names of the images indicate that the band is working on '5ths' (click images to enlarge):

edgeordiefifths edwhiterosefifths jonnyworldfifths2
'5ths Reversed' is a title listed in the credits for episodes of 'The Most Gigantic Lying Mouth of All Time'. It could either refer to the electronic track following the 'Lying Mouth Theme', or the ambient track playing in the background of Chieftan Mews segments.
'Piano 5ths' is a track from 'MiniDiscs [HACKED]', a leaked assortment of audio from the OK Computer era. It is a simple track with piano and synths. The relation of this to the other '5ths' tracks (if any) is unknown.