Perhaps you are familiar with what the SAAC has been able to do in maintaining the histories of Shelbys. That simply isn't possible with ZL1 Camaros. By the time the VINs were known, the cars were 12 years old. Back in the '60s, some states titled cars, some did not. In some states, cars were registered by county. Even when you could run a VIN nationwide, it wouldn't query those states. Some cars were raced from new, never titled or registered. Several others were stolen while on MSO. At least two left the country.
When the VINs became known, several people, myself included, started trying to locate the cars. Fortunately, many were found and saved from the normal ravages of time. Once the original engine was removed, often the cars' identity was lost and the owner had no idea of their cars' original configuration. As recently as a few years ago, a long-term owner of a '69 Camaro race car was very surprised to hear the good news.
I track the existing cars as best I can. Often, cars trade hands privately. Some ZL1 owners like to be active with their cars. I have found that most not only do not, they don't care to be blabbed about. So, I respect their privacy. Because of that, information often comes my way.
Searches today are virtually impossible for the average person so the history of the 20 unknown ZL1 Camaros is probably lost in time.