(August 1960), DC Comics "The Three Super-Heroes" by Jerry Siegel (writer) and Jim Mooney (artist)
reprinted (with major differences) in ACTION COMICS # 334 (March 1966)
This is a very odd little back-up story that if nothing else illustrates how Mort Weisinger and his writers kind of made up the Legion of Super-Heroes schtick as they went along.
Linda (Supergirl) Lee is riding on the orphanage bus to a fair at the Superman Museum in Metropolis. The bus is going to be delayed at a bridge crossing, but Linda decides that risking having the world learn of her existence must take second place in importance to getting to the museum on time, so she dashes out the emergency exit, shoves the boat down into the water sufficiently that it clears the bridge with ease, and then pops back onto the bus. This entire two- page sequence was chopped out of the story when it was reprinted, and no great loss.
At the museum, Linda meets Jimmy Olsen -- well, it certainly LOOKS that way. A red-haired kid with a green jacket and bow tie hanging out at the Superman Museum in a Mort Weisinger comic book -- 99 times out of 100, that kid is Jimmy Olsen. Not this time, though -- turns out to be Lightning Lad in disguise, putting Linda through the usual pointless Legion hazing ritual where the Legionnaires cause imminent catastrophes and then avert them while the hero being hazed has to stand there and be taunted. Gee, what a fun-loving group of scamps!
Well, Linda puts up with the taunts from Saturn Girl and then Cosmic Boy, similarly dressed in street garb, until they decide to clue Linda in on the joke. Here's where it gets good.
The three scalawags introduce themselves as the Legion of Super- Heroes, which helps jog a chord in Supergirl's super-memory, who remembers Superman telling her about a club he joined as Superboy.
Supergirl: "That's it! You're the Cosmic Boy... Saturn Girl... and Lightning Lad he knew!"
Lightning Lad: "Not the ones he knew, although we have the same names..."
Saturn Girl: "We are the CHILDREN of the three young super-heroes who befriended Superboy! We are carrying on the Legion's traditions."
Everybody catch that -- these are second-generation Legionnaires, the children of the originals who knew Superboy. In the reprint, though -- which came along 6 years later, by which time DC had pretty much established how all the time paradoxes work so that Supergirl and Superboy could BOTH be in the Legion at the same time -- both LL's and SG's dialogue balloons are completely rewritten:
LL: "You're hitting on all cylinders, Supergirl!" (why exactly would they still have cylinders in the 30th Century?)
SG: "We've admired Superboy... and you! That's why we made this trip!"
Either way, Supergirl is clearly dealing with three nutjobs who don't mind threatening innocent bystanders just to while the time away before they get around to inviting Supergirl to join their club.
Oh, but hold on now -- just because they came all the way back to the 20th Century and have admired the legendary Supergirl for a millennium doesn't mean she's a lock to join. She has to compete against other applicants for membership. The thing is -- Supergirl was INVITED to join, not invited to apply, since Cosmic Boy clearly said, in both the original and the reprint, "Supergirl, would you be interested in visiting our future world and joining the Legion of Super-Heroes?"
Well, wouldn't you know, she ends up digging an underground tunnel through the entire Earth to divert traffic as her obligatory super- stunt to prove how worthy she is of joining these other goofballs, but as luck has it, she gets exposed to Red Kryptonite and ages a few years, just old enough to no longer be a teenager, so she's out. Now, why she didn't just wait for the Red K to wear off, as it always does, we don't know, except this way, Mort & Co. get to do yet another "Supergirl joins the Legion story" in a few months. Whatever.