Here is a post from John in 2005 on the subject:
JohnZ Jul 13th, 05
"This is a great debate subject. I have the GM drawing for 3877660, released in 1965, and it says "Cast manufacturer's identification, part number and General Motors trade mark location optional." Three different suppliers made this part over the years it was in use, and each one put the part number in a different place. In NCRS Corvette judging, the pattern that emerged after observing thousands of known-original cars indicates that those used through 1969 had the number on the flange, and those used from late 1969-up had it on the top of the outlet neck. The current reproduction, made by Paragon, has it on the outlet neck. My original 02D '69 Z/28 has it on the flange."