Oldest would be Paul Bley sessions that resulted in ECM 1003, dating to 1963 or thereabouts. They were originally slated for Savoy, I believe, but were not issued until Paul Bley with Gary Peacock.
They also reissued the Giuffre sessions from '61 that resulted in Thesis and Fusion.
I'm pretty sure that there is more music from the Just Music date that resulted in ECM 1002, which was a reissue of a privately-pressed LP recorded in 1969 with Alfred Harth, Thomas Cremer, Thomas Stoewsand and others. But it would be in Harth's possession. Eicher has worked pretty closely with musicians to ensure some amount of "perfection," so the presence of rejected takes seems sort of anathema to what ECM is "about."