I do think that MSG and black pepper (maybe even some white) are an important part of the recipe.
But I think that the flavor (the one or two spices, that are very closely related) that make it truly unique—the cornerstone of the recipe—especially the Kentucky Fried Chicken of 30 years ago and prior, has been overlooked by almost everyone.
Hint: They're indirectly mentioned in the Big Secrets book.
I can remember James Beard picking up on the flavor in a different account, many years ago.
I can't find a reference to it now, but when I read it I remember thinking "YES!"
It's odd that he's not quoted as mentioning it in the "Big Secrets" book—which makes me wonder how accurate the Big Secrets book is.