Here is a quote from a post over on CRG from 2012. Yes, it is my post from this thread:
http://www.camaros.org/forum/index.php?topic=4484.15
I have identified at least 4 different versions of these valve covers in aluminum, not including the black wrinkle finish ones.
The earliest without drippers are the 418. They were used in 69 only. I believe the second design, with drippers, but same exact appearance, appeared late 69. Both were dull finish. Part numbers on the first ones with drippers ends in 417 instead of 418.
They are exactly the same on the outside, including the reliefs cut on the inside edges to clear alumiinum manifolds. Those reliefs are each 3 inches long. The next set has a part number ending in 208 and 207 for rih and lh. They are the most common and I believe most were polished originally. Easiest way to id them on a car is the reliefs. One is 3.5 inches and the other is obviously longer at about 5 inches. The reliefs on the early ones are sometimes a bit crude, and appear to have been cut by hand. The later ones are always perfect.
Adding the the confusion is the number of early valve covers, both with and without drippers, that were sanded and polished. Eddie, you may have a set that someone polished. If they have the short reliefs and end in part number 417, I think there is a very good chance that they are the originals. I just took a set off a very original 350 horse 350 assembled 10 18 69. This engine is so original the main and rod bearings are dated 9 of 69! Never been apart.
NOW; I realize several experts have indicated that NO Z/28 received the dripper 417 valve covers. However, you must realize that there has been published data indicating what valve covers came on a 69 Z/28. Anyone wanting to get theirs restored would of course, look for the early ones that had received the "stamp of approval".
I have torn down at least one late Z/28 with those 417 dripper valve covers and one 70 Corvette assembled Sept 69 with those valve covers. Not sure when the transition started.