French Maker's Marks

[bijoux.expert]

Hello,
I'm presently working on a new book, Maker's Marks of the Great French Jewellery Houses and their Workshops.
While it's not difficult to find all the marks of the major houses, it's virtually impossible to find ALL of the workshops (or subcontractors) who made silverware and jewellery for these houses.
I have over 430 at the moment, but there could be hundreds more still to be found. In an ideal world, I need a clear photo of the maker's mark, so that I can make an accurate drawing, the date of activity of the workshop, a history of the founder and family, a list of all the important houses for which the workshop worked and, ideally, the exact period in which it worked for each of the houses.
Much of the information I find is partial and even contradictory. The hardest to find are those who worked before 1930 and those who work today, as they're often bound by confidentiality agreements.
Any information, no matter how small, is welcome.

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Thanks !