i have a feeling i am not going to be any help for you,
but with a very long exposure ( i mean hours )
will a modern direct positive paper yield a positive image ?
depending on the kind of day and the light, i have gotten very good
in-camera negative images on regular photo paper ( not direct positive )
i have exposed 30 minutes to 3 hours and depending on my subject i get
more blue light, more exposure, less time ...
perhaps his direct positive was the same way, a very long exposure , and a fix bath.
my last 2 images in my gallery are from negatives made this way.
no-chemistry, just sunlight and photo paper.