The two letters after 9511 are the ECL code and would be specific to each model of car, and what "standard" axle ratio was getting replaced. Each ECL points to a different component parts sheet, which lists what parts "go away" (the standard axle, the standard speedo driven gear, and possibly the transmission if a different internal speedo drive gear was necessary for the 4.88), and what parts get added to replace those items (plus a speedo reducer if they used one for that combo).

Camaros, Chevelles and Novas would each have different axle part numbers, so would require a different ECL suffix code for each car. And if COPO 9511 covered both 4.56 and 4.88 (and 4.10 for cars that had no 4.10 RPO available), then there would be a different sub-code (the second letter) for each of those ratios.

Anyone have any Camaro or Chevelle window sticker examples with 9511?? on them?

here is a post I did on the black Yenko sold 69 Nova that has a BV code on the POP