What does code PO mean?

I found the build sheet for my 1970 Wildcat 2 door hardtop. Code for posi rear is PO. Car is a 1970 Wildcat 455 with factory a/c. Can someone tell me what that code refers to ----> axle ratio ?

 

Looked at the Dove book. Shows PO as a 2:78 Posi.

 

Thanks Fred. A little low but maybe good in these times of $3.50 a gallon gas.