Actually, I lied.... I went back over the cars photos and parts and did find a long strap that "could" be it. About 1/2" wide and 6-7" long, folded pretty much in half. Rubber dipped cover like the J bracket that hold the harness to the dash support behind the cluster.
It connects to the stud at the front of the fresh air vent that is not used by the vent housing. Odd location as it pulls the whole harness far forward and makes it diffucult to connect the instrument cluster. It was in a April build 289/C4 Cal Special... nothing like it in any of the big blocks, so not sure if that is relevant, which would not make much sense if it was different for them as the harness and dash are the same...