50s Cap is one of those retcons done right that rarely happen. You see, In The Real World, Captain America's comics just sort of limped along after WWII, eventually ending like almost every other superhero. Then Stan Lee got creative control of Marvel, and brought Cap back . . . revealing in an untold story that he'd been MIA since WWII ended, and Bucky was dead, Steve Trevor was a man out of time, and that was just the status quo for years . . .
Then someone (probably Roy Thomas, or some other Golden Age ascended fanboy) remembered that Cap's comic book continued after the war . . . but if Captain America was MIA since WWII ended, then who busted commies in the 50s?
I think an issue of What-If brought about the possibilities that other people wore the Captain America outfit after Steve disappeared, which eventually became canon (like the "G-Men" eventually became Agents of Atlas). And the rest was history.
But good retcons happens really rarely, and what's startling here is that it's a retcon of a retcon thus making it even less likely to be good. That both retcons are actually good? Astounding.