It's a very sharp badge - but I don't get it . . .
As I understand it Z28 started out as an RPO code for a 302 CI v-thingie for use in 305 CI limited SCCA events. It was so popular they began to badge cars that had the option.
Similarly Z26 is an RPO code for a late 70s early 80s 'Rally' optioned Nova . . .