Superman 300 (June 1976) is the one and only appearance of an alternate Superman, who lands on Earth in 1976, in a story by Bates, Maggin, Swan and Oskner.
While the story will vary greatly from the established Superman lore, the Krypton opening, with Jor-El and Lara firing off their son in a rocket, is basically identical.
The US and the Soviets both race to intercept the incoming alien craft in a destructive competition. It’s an American, named Kent, who winds up getting to the rocket first.
But the boy is raised in secret by the military, and though he adopts the identity of Skyboy, his existence is not known to the world at large. Meanwhile, we see that in this world, by 1990, there are buildings a mile high and impenetrable domes.
When the president of the US decides to reveal Skyboy’s existence, this sparks a nuclear war with the Soviets. But Skyboy destroys all the weapons.
As a result by 2001 there are all manner of advances that our world never achieved, like floating chairs and holographic tv (although we’re pretty close to that one).
Changing his name to Superman, he makes his public debut, revealing that he had prevented the nuclear war.
It’s not a bad story, but it’s really only an introduction. And with no further appearances, a somewhat irrelevant one.