harrison2 - SG refers to specific gravity. This is a measure of density. Each metal has a different SG which can be determined by weighing the object in air and then in water. The difference (weight in air minus weight in water) is devided by the weight in air to get a number. Gold is 19.32, Silver 10.50, Platinum 21.45 and copper 8.96. The SG of various metalic alloys also varies so that the more copper is mixed with silver the lower the SG of the alloy.
What I meant by a SG above 10.0 was that when you have silver that is roughly alloyed for coins under crude war time circumstances - you tend to get a range of purities of silver that result in a range of Specific Gravities. Mexican coin silver 0.903 fine has a Specific Gravity of about 10.3 not the 10.5 of pure silver. So what I would be looking for is a coin with silver of 10.0 SG or HIGHER. White metal alloys with nickel and copper are often in the 8.9 range.