The sticking point here is that later revisions look almost identical to the earlier ones, while at the same time the corrections look too good to have been pro

The sticking point here is that later revisions look almost identical to the earlier ones, while at the same time the corrections look too good to have been produced by the then-common "white-out / replace / Xerox" method (page 5). So they might have been retyped, and since doing that using a typist would have been prohibitively expensive, it might have been done using a computer - however the authors haven't found any evidence pointing to that, so they avoid stating it clearly...