But back to the bacic qustion that started it all....

What does 12 DWT mean as part of a hallmark of plated silver.

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The following is an excerpt from The Household Cyclopedia of General
Information published in 1881.

The assay of silver


The general method of examining the purity of silver is by mixing it with a
quantity of lead proportionate to the supposed portion of alloy; by testing
this mixture, and afterwards weighing the remaining button of silver. This
is the same process as refining silver by cupellation.

It is supposed that the mass of silver to be examined consists of 12 equal
parts, called penny-weights, so that if an ingot weighs 1 oz., each of the
parts will be 1-12th oz. Hence, if the mass of silver be pure, it is called
silver of 12 dwts.; if it contain 1-12th of its weight of alloy, it is
called silver of 11 dwts.; if 2-12ths of its weight be alloy it is called
silver of 10 dwts, which parts of pure silver are galled 5 dwts. It must be
observed here that assayers give the name cwt. to a weight equal to 24 real
grs., which must not be confounded with their ideal weight. The assayers'
grs. are galled fine grs. An ingot of fine silver, or silver of 12 dwts.,
contains, then, 288 fine grs.; if this ingot contain 1-288th of alloy, it is
said to be silver of 11 dwts. and 23 grs.; if it contain 4-288th of alloy,
it is said to be 11 dwts, 20 grs., etc. Now a certain real weight must be
taken to represent the assay-weights; for instance, 36 real grs. represent
12 fine dwts.; this is subdivided into a sufficient number of other smaller
weights, which also represent fractions of fine dwts. and grs. Thus, 18 real
grs. represent 6 fine dwts, 3 real grs. represent 1 fine dwt., or 24 grs.; 1
1/2 real grs. represent 12 grs.; 1- 32d of a real gr. represents 1/4 of a
fine gr., which is only 1-752d part of a mass of 12 cwt.