Here is a link to what some Corvette people have to say
http://forums.corvetteforum.com/c1-a...-original.html
The best way to search is to look for "Lester Corvette valve covers". Lester must have been the casting company that made them. I sold a pair, new in the box, a couple of months ago for 175. They had Lester cast on the inside of the covers and the "O" that had the center mostly filled in. Evidently some of the earlier ones (around 1966 ? ) had the casting line but still had the open centered "O".