As to the correct engine paint, it was quite abundant in the mid 70's, at least in the Baltimore area. The most common brand was NAPA's Martian Seniour, and there was never a shortage. If I recall, it even listed the years because of the other blue on the market, and the napa paint matched exactly.
As far as questions why things are different, the not having it on hand is a good possibility. If you work on hundereds of the same model & year cars, it's more than apparent that there are lots of differences because if something isn't availble when the line is running, they'll use anything that they can to get the cars down the line. Some may be worse than others, but it applies to all cars.
I wasn't spray bombing engines yet in the 70's. With Merv's great experience and memory we now know what was going on. As he a stated, the spray cans matched perfectly. This points directly at there being one color that was predominant. I've worked on many GM assembly lines and I have to agree that there will be a small number of engines that will vary due to 'get er done' mentality.
Pictures are just plain invaluable. With some serious scutiny and a little deductive reasoning you can tell a lot from a picture. Like the stud being painted on Yellowbird's engine that points at the riser being installed and then painted. Actually, it looks like it was painted, installed and then painted again. That would tend to disqualify at least some of the paint as 'factory' paint. Another great example of the value of a picture and reasoning is found in the picture of Rodericks car that is said to have 10,000 miles on it and then totaled. As the story goes it was walled in a garage and never touched again. One of the most revealing things about that engine picture is that the water pump appears to have been painted separately from the rest of the engine and at a later date. If it isn't the original water pump then the original did not last very long.
