Thanks, Mark. I don't own a copy of _Examples_, and my other source was a little dubious, though he did use Agfa Isopan in some of his earlier work. So much for the longer exposure theory. I think I've seen the print at some point, though I don't recall where. Maybe MOMA has one in its general collection, or had one on loan at some point.
I have some Super-XX, so I could try, though it is probably not the same Super-XX that Adams would have used in 1939.