Following the discipline published in this forum by a 1960s sound engineer re DM's[[Duplicate Masters at Detroit) which probably carried over to LA studios the Master Number was allocated at the end of sessions when it was decided to keep the track for quality assessment and acetates were made. We have three completed acetates from Bob Crewe's final sessions[Nov 1973- they are on You Tube]....which also resulted in 'My Eyes Adored You' and the Anthology CD set from 2008 includes 2 previously unreleased versions clearly salvaged as complete from Master Tapes. So researching these whilst revealing unfinished work will more than probably result in 'unreleased' Masters. There is just too much evidence that a good number will be completed Masters. Why undertake the admin work of recording a track on the system if it is not cleared by the producer and engineer for next stage assessment. There is plenty of evidence of titles probably unfinished....but the Tape research project of KEY Tapes identified in the Artists Cards shows many cases of copying to other 16 track and Stereo Mixes. Russ Terrana and others would have only done this with completed Masters.