Keith,
It should be the SEQ# from the GM Body or Chasis Broadcast copy. Did you find either one of these? The SEQ# would be on top of the black paint.
The
69 L78 Chevelle on ebay has a Body Broadcast Copy with SEQ# 246. 246 is reproduced on the Differential Cover and the spare. These are some examples where SEQ#
might appear on a car. Greg