Last issue of the Legion's run in Action Comics.
This story is often mis-remembered as an alternate version of the Legion. In reality, it was a hoax, with Brainiac 5 and Sun Boy playing parts of the male Prince Projectur and Saturn Lad. Saturn Girl presented the false "alternate world" theory as part of the hoax.
This misconception is perhaps strengthened by the appearance of a Legion Headquarters that's totally non-canonical.