I've seen Clayton, the Weavers, Pete Seeger, Bill Monroe and country singer Billy Grammer (who had a hit with it) listed as co-authors of this song---all on the same line in the liner notes to Monroe's Decca LP. I suspect all of these people added a verse or two to it and changed it. But Clayton may have been the one who found the chorus in fragmentary form and wrote the first actual verses and then used his found verse as the chorus.
I think Monroe added this verse, at least, to his bluegrass version of the song:
Papa writes to Johnny---Johnny can't come home,
Johnny can't come home--Johnny can't come home,
Papa writes to Johnny and Johnny can't come home,
He's been down on the chain gang too long.