Lets see if you can tell me when is a pound not 16 ounces?  Answer later if anyone doesn't know.Here is a silver trading unit for silver 31.1 grams .999 fine silver

Troy weight is a system of units of mass customarily used for precious metals and gemstones. There are 12 troy ounces per troy pound, rather than the 16 ounces per pound found in the more common avoirdupois system. The troy ounce is 480 grains, compared with the avoirdupois ounce, which is 437½ grains. Both systems use the same grain defined by the international yard and pound agreement of 1959 as exactly 0.06479891 gram. Although troy ounces are still used to weigh gold, silver, and gemstones, troy weight is no longer used in most other applications.