Daguerreotype plate sign

There was a sign on a part of daguerreotype plates to show the
quality of plates. This is a number, usually 40 but sometimes 30 or
20 and in a few cases together with a letter "M". When this number
eg. 40 - about one part of my sources this means: 4% of the whole
weight of plate is silver (40 units from 1000). But about other
sources the meaning is: 1 unit of silver and 39 units of other
material (the copper plate). This sign was used at first by French
producers and later in North America also, but not in other
countries of Europe.

Which one is the right version? Is it possible: two systems with the
same signs in the same time? I don't tell you the sources to keep
the jury independent :-)

Balint Flesch

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                  Conservation DistList Instance 10:21
                  Distributed: Friday, August 23, 1996
                       Message Id: cdl-10-21-007
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