Required option is different than being part of and engineered option (COPO). All of the Z10, and Z11 were required to have option ZL2, and it did show up on the window sticker as a separate line item. I would think that any car that had an option called out on the window sticker as a separate line item would cause that RPO to be added to the total number of units produced for that option. Z28's required a muncie transmission, and power disc brakes but they weren't part of RPOZ28, which was essentially the 302 engine, and you paid for them as an extra option item. I don't know if the COPOs were listed as a single line item ie 9557, with a list of peices parts that came under that option, or if they listed each separate item like ZL2, M22,L72, etc line by line.