When the "lost" movie script that Betty and William have been looking for for half the episode suddenly appears in William's nightstand drawer, Betty surprises William with another copy from her nightstand drawer and tells him that she had made a carbon copy of it.
You can't make a carbon copy of a document after the fact. For Betty to have made a carbon copy of the script, she would have had to feed two sheets of paper, with a sheet of carbon paper between them, every time she loaded a new page into the typewriter. On a script that could have approached or exceeded 100 pages, written over many months and completed on board, it would have been impossible for William to not know that a second copy of the script existed.
You can't make a carbon copy of a document after the fact. For Betty to have made a carbon copy of the script, she would have had to feed two sheets of paper, with a sheet of carbon paper between them, every time she loaded a new page into the typewriter. On a script that could have approached or exceeded 100 pages, written over many months and completed on board, it would have been impossible for William to not know that a second copy of the script existed.