Can we talk about some of the various groups that have been retroactively referred to as "the Avengers of the '50s"? I know there have been at least two.
Sure thing.
The "Avengers of the 1950s" are an interesting little historical footnote that, until recently, have been hidden behind layers of government-imposed secrecy.
As you say, there have been multiple groups that have, somewhat retroactively, been given that title - but of those, only two are noteworthy enough to cover here.
And what's more noteworthy of both of them that, being born of the Cold War, neither are what we'd call conventional superhero teams in the vein of the All-Winners Squad before them.
(photo of the so-called "Avengers 1959", edited to remove background for confidential reasons. Left to right: Victor Creed, Namora, Dominic Fortune, Dum Dum Dugan, Ulysses Bloodstone, Sergei Kravinoff.)
The so-called "Avengers of 1959" were one of these outfits - less a superhero team and more a covert government operation, a predecessor to the SHIELD Super-Agents more than anything, albeit one whose members had quite the checkered past.
On the orders of President Eisenhower, Nick Fury recruited a crack team for a covert operation against a Red Skull imposter and a band of neo-Nazis. This team consisted of big-game hunter Sergei Kravinoff (who'd later be a persistent thorn in Spider-Man's side), feral mutant Victor Creed (alias Sabretooth, wanted for murder in several jurisdictions), soldier of fortune Dominic Fortune, professional mercenary Ernst Sablinova, alias the Silver Sable, Atlantean princess Namora, and immortal monster hunter Ulysses Bloodstone.
The only unambiguously good egg on this team was Fury's old war buddy Dum Dum Dugan. The rest ranged from morally grey to outright criminal, only held in check by promises of a paycheck and Fury's tight leash.
Unsurprisingly, they disbanded after only two missions - the one against the fake Skull and a second, more consequential operation that saved the life of King T'Chaka of Wakanda, father of the current Black Panther.
The whole group was a powder keg about to explode, and it's a miracle it lasted as long as it did. In contrast, the other group that went by that name at the time...
(Modern-day artist's rendition of the "Agents of Atlas", recreated from newspaper descriptions.)
Shortly before Fury assembled his team, FBI agent James Woo was given special authorization to assemble "Department Zero", a team of operatives tasked with rescuing Eisenhower from the Chinese terrorist Plan Chu, alias the Golden Claw (period documents say "Yellow Claw", but consensus is that this is a racist mistranslation).
Woo recruited a group of operatives to rescue Eisenhower including Robert Grayson, alias Marvel Boy, Ken Hale, the Gorilla-Man, and Venus - reportedly the Roman goddess herself. Namora was asked to join the team but refused, providing the team with the salvaged Human Robot as a replacement.
Department Zero's mission was successful, but in the end, the United States government shuttered their operation and classified all related material. The group seems to have reformed as of late, operating as the "Agents of Atlas", although details on them are scarce. (Whether this team is one and the same as the Asia-based team that formed in the wake of Malekith's attack on Earth is unknown.)
