With the online research I've done, I'm understanding that there wasn't any strict diet plans for Golden Era Bodybuilders. They all differed from individual to individual.
I like to follow a Steve Reeves type diet - fruits, veg, nuts, seeds, eggs, fish & lots of water. Whereas I read that Reg Park would eat sausage, bacon, fried eggs & lots of bread for breakfast.
It sounds to me that they eat home cooked meals, with no refined sugar or white flour. They would eat foods that they grew up with that works for them.