1. Try what I said--exposing to direct sunlight for a day or two. If it's fungus that will make a big difference.
2. The Jamin/Darlot period was far earlier than this lens and involved Petzvals. This lens is much later than that. For starters rectilinears weren't much in use until about 1866 and later. This one has later construction so early 1890s is a good guess.
Kent in SD
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