I looked at this issue a number of years ago and I wound up convinced that GM did a supplier change around mid 1968. Anyway the image below is correct for 1967 and you can see the contract supplier mark right on the casting on the lip along with the run number.
The later castings are up on the neck typically with a casting run number only as we are accustomed to seeing on the later cars, and the reproductions are also made off of this last version.
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