After the death of George Reeves, the star of Adventures of Superman (1952), the producers considered replacing him with Alyn. The plan never materialized and the series was abruptly ended.
At 37 years old, he was the joint oldest actor to make his live-action debut as Clark Kent / Superman. George Reeves was the same age when he made his first appearance in the role in Superman and the Mole-Men (1951).
He has appeared in one film that has been selected for the National Film Registry by the Library of Congress as being "culturally, historically or aesthetically" significant: Superman (1978).