Was RPO PL5 a mandatory option for the COPO cars?

[Jeff H]

I've noticed that it shows up on all the window stickers I've seen for COPO cars. But I also noticed that the 4.10 HD rear showed up as an extra cost option on 1 COPO but appears to be standard on another. Was there a change in what was included with the COPO 9561 part way through the year. The later of the 2 cars had the 4.10 rear as a seperate option.