I can't find any reference to a 2012 Masterpieces set on the RAM website (or anywhere else), so it would appear that none was forthcoming..
I recently acquired a 2006 set, and sadly, the plaque is not silver

It's a soft white metal, non magnetic, SG between 7 & 8 (from memory). my guess is a zinc based alloy (??)
I'd say it's a pretty safe bet that the 2007 plaque is the same.
re the 2008 - '10 sets, I don't have any of these (yet), & haven't sighted a COA, but my sources (incl. McDonald) indicate that these have a nominal mass (per coin) of 36.31gm.. Is this correct?