I think I finally solved the '54' meaning.
Reverse it to '45'.
Look at the intro page for the "The Superman of 2965" story.
According to the tombstone of Superman I, he was born in 1920. That would make him 45 at the time of the "The Superman of 2965" story.
'54' is '45' backwards so it's Superman's supposed age in 1965 if he was born in 1920 as that tombstone states.