Hallmarking TwoTone metal

Thanks Ian, but we do have a system in place in the USA since
around 1900, there was a post that went over it a bit
http://www.ganoksin.com/orchid/archive/200505/msg00338.htm 

Thanks Tim, I had missed that and, from the recent posts, it seems a
lot of other folks had too. This, of course, is basically exactly
the same as the UK hallmarking legislation and, as far as I can see,
doesn’t allow for the multiple marking of pieces with different
metal standards. The section which says ‘you cannot quality mark a
piece of gold with a higher quality than that of which it can be
assayed.’ surely means that you can only mark a piece with one mark
indicating the lowest quality of precious metal in the piece. That
is, the standard mark which would represent the overall quality of
metal if the piece were melted and reduced to an ingot. This is the
way it was described to me by the head of the Sheffield Assay Office
laboratory. The only exception to this is where the different grades
of metal are on separate parts of the piece which can easily be
taken apart to separate the metals and melt each part separately.
Where the silver or gold cannot be individually assayed it may only
be sold as ‘white metal’ or ‘yellow metal’ and it would then be up
to the buyer to take a chance on the exact quality of the metal
used.

Best Wishes
Ian
Ian W. Wright
Sheffield UK