Another suspect detail in this story: Frederick says "hanging on a rack was the Superman costume" and one of the men told him, "If it fits, you’ve got the part." This story makes it sound as if the producers were too cheap to care about an actor's abilities so long as they didn't have to spend money making a new costume. The trouble is, what's pictured is NOT the Superman costume.
That's my point. It seems pretty clear that Frederick
thought it was the Superman costume (either at the time or decades later, probably just thinking of "superman" as a generic term for costumed flying hero), but it's obviously not the "real" Superman costume. So did the producers every actually say, "This is the *Superman* costume"? Or did they just say, "try on this outfit and if it fits, you've got the job." I'm guessing the latter, that the postulated episodes were about a different but similar hero, and that Frederick wrote "Clark Kent" and "Superman" on the photos himself.
I wish the photo were bigger, it's hard to make out. Sort of looks like it could be a recycled Flash Gordon costume or something.