Regarding the other holder, perhaps containing unexposed film...
While it might be fun to shoot the 100 year old film it may also be that the film has been exposed (maybe the photographer didn't indicate properly).
When I am confronted with a questionable piece of film I always err on the side of assuming the film is exposed. I think it is better to waste/develop a sheet of unexposed film than to waste two shots, the original and the later one.
You can make your own dry plates if you want to experiment with the use of glass. You can't very well undo the double exposure if the glass had been used.
Just my 2 cents worth.
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