Superboy Vol. 1 (1949-1976) / Superboy and the Legion of Super-Heroes (1976-1979) #206

Plot

Superboy encounters dead Legionnaires Ferro Lad and Invisible Kid in Smallville. They tell him they are testing themselves to see if they still have what it takes to be in the Legion. But Superboy knows they are experimental clones who only survive for 2 days.