You looking at a UK/Euro or US spec car? C7 Z06s don't have a/the Z51 option. They may have the optional carbon ceramic brakes (Brembo), or the Z07 Performance package, which includes the carbon ceramic brakes, Z07 specific Magnetic Ride Control and "other" things (ground effects/Cup 2 tyres etc).
RPO option sticker should be under the carpet in the trunk on the right where the battery is:
Can't find anything on the interwebbie about it after a quick look? Do they have them on a C7? Thanks.
www.corvetteclub.org.uk