Chick, I try not to say never which is why I don't participate in judging (just WAY TOO MUCH DRAMA), so it's very possible there were mirrors with little to no parting line. However, that being said, the process of die casting leads to quality issues after xxx number of castings have been done. Dies wear, and especially dies where you have to place two or more dies together to form one part. The mirror base consisted of two separate dies, joined together at the parting line and then the metal poured into the die. While I don't know this for sure, I'm fairly certain the GM directive stipulated that the excess slag be ground off. This would have probably included any excess slag at the parting line. Especially at the top where it's easily seen. Did they always do this? Apparently not since original mirrors have been observed with a HUGE parting line (up to a few millimeters in fact)!
Yes, I'm sure there were many sets of dies, and lots of replacements once the originals became too worn to use. This same exact base was used on the Camaro, Chevelle, Nova, Full size... and even other GM divisions for several years, so it stands to reason there would be varying degree of parting lines due to the number of dies used, the age of a particular set of dies and the degree at which the metal worker cared!!
I've attached a picture of a base with the "typical" parting line usually seen (although I've seen a LOT worse than this one!), and another picture of a base with a very wide parting line as shown if you were looking forward seated in the driver's seat.
However, all this being said, if you're having your car judged and the rules say there should be a parting line, then there should be a parting line! But again, I don't get involved in judging since I don't need the headaches!
Ed