Z-28 was an order code called RPO.....Regular Production Order. Instead of Chevrolet making you choose each individual option, they used a special code like Z-28. The code denoted the individual parts like dual exhaust, special handling package, special engine, special wheels etc. The customer would order a Z-28 and everything that was under that option code would be placed on their car at the time it was built. You can see Chevrolet still uses these codes on many cars and trucks today, like Z71 on their pick ups and SUV's. This still denotes a special package that has been placed on that vehicle at the time of build.