"Paul Clayton and the Folksong Revival," by 'Catter Bob Coltman, has this to say:

"Trying to over come the banality of the widely known "Jesse James," he put a new chorus - 'I don't know where my poor old Jesse's gone' - to barely reworked verses and copyrighted that."

Bob doesn't tell us anything about Thee Phillips, however - or whether he was Clayton's source for the "new" chorus. It would certainly appear so - especially if Deckman's recollected date of 1954 is correct.