Superman I dies after being "attacked by an Energy Vampire from space" and the "remnants of his super-powers" are "amplified and transfused into a bio-copy of the original." This copy is known as Superman II to future historians.
Superman learns of these events when he is accidentally transported by historians to the 24th century of a parallel Earth along with George Washington, Abraham Lincoln, and George Custer (Act No. 399/1, Apr 1971: "Superman, You're Dead ...Dead ...Dead!"). Upon his return to the 20th century, Superman wonders if the Washington, Lincoln and Custer he has encountered are from his Earth or a parallel-world, since all three relate histories that are markedly different from their Earth-1 counterparts. He ultimately decides the answer is unknowable.