Head identification

I have a pair of Big block Chevy heads that I bought a couple years ago from a local engine builder. Story on them is that they are experimental open chamber heads from 1968 that never should have left GM's possesion. Bill Jenkins had them and an employee of his sold them to the engine builder after his death. Neither me nor a few friends have been able to trace the serial number. They are for sale as well since I have decided to build an early hemi instead of a BBC. Thanks

Markings on the intake runners are the serial number 0-306106
GM10
S-8
HI-PERF