The big change was when they changed engine plants they also changed some of the parts, the Mound Road plant went away from the valve cover using the large wire protector on the passenger side cover and instead wen with the 3 small bendable wire retention tabs. I've seen a couple exceptions that were probably due to a small amount of inventory crossover, but for the most part, a blue engine should have the 3 small tabs. Also, the intake manifold casting, while having the same casting number ending in 100, was a new casting and is shaped differently in many areas. Coils were also different, Windsor coils had "Made in Canada" cast into the snout, Mound Road had "Made In USA" on them. There were more differences as well concerning parts and paint coverage details.

Here are some typical "blue engine" valve covers. They remained the same for 1972.