The villains unveil themselves in Avengers #126 (1974):
But allow us at Slay Monstrobot to embiggen that Editor's Note for you:
Even worse? No No-Prize was ever given!!
Only one letter in Avengers #130, from Chuck Mabry of Washington, D.C., even mentioned the contest...
Now, it's possible that Marvel did award Mr. Mabry that No-Prize, and just didn't bother to actually mention it. Or maybe they did award the No-Prize to someone else, but didn't actually print the letter, or mention it in any other way. Although if you're going to cock-up the premise of your story like that, and then have the balls to use a reader contest to try and cover your ass while hyping it as a "Mighty Marvel First," than I think you're sort of ethically obligated to honor that, and explicitly tell the fans who, if anyone, won.
Or maybe, just maybe, there's a No-Prize laying around the Marvel offices, waiting to be awarded!!
So I suggest you all bombard Marvel with letters explaining how Klaw and Solarr got together. Write away, my pretties!!