Superboy #68 (Nov 1960) features the next appearance of a young Lex Luthor in a Superboy story. Superboy eventually defeats the “Army of Living Kryptonite Men” by getting help another L.L: Lightning Lad from the Legion of Super-Heroes.
Lex learns of the existence of the Legion and theorizes that a Legion of Super-Villains must also exist in the future. This is the first time I’ve noticed DC trying to fore-shadow a bigger arc and future story directions across its books outside of the letters column.