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.
