The pedant in me wants to point out that the external tank is not "Endeavour's" because the external tanks were not recovered after flights so that one has never been on a mission and could have been a tank for any shuttle mission.
It sure is Endeavour’s now, for hopefully many decades longer than any other tank.
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.
The external tank destined for display at the California Science Center is
ET-94, the last flight-qualified external tank in existence. This lightweight
tank (LWT) — donated to the Science Center by NASA — was ordered to support
science missions for space shuttle Columbia.
I know that the ETs were updated after Columbia was lost, but I did not realize that a complete tank was built before the changes, yet not flown.
[0] https://europeanspaceflight.com/the-case-of-the-missing-vega...