I suppose that the external tanks may have been commissioned for specific missions, like Atlas IV and Delta V rockets were. Maybe this tank was commissioned for a mission that never flew, like the current museum Saturn Vs.

I remember, possibly incorrectly, that there was a contingency rescue mission stacked but not flown, though I think that the stack did fly on a NASA designated mission (as opposed to the usual Congress designated missions). Though I think that was Atlantis.