The decoration on the case back may be partly die struck, but there will be a few areas wher the decoration has been applied by hand engraving. Hand engraving differs from machine engraving in that surface metal is actually cut away by hard, sharp steel engraving tools manipulated by hand. If an item is gold filled, very often the graver will cut through the surface layer of gold to expose base metal. A tiny bead of nitric acid in a deep spot in the pattern which may already have turned black (oxidized) might react to the nitric acid. Another method that is used is called a “streak” test in which test needles of varying karat alloys are rubbed onto an abrasive Arkansas stone, then the item on test is abraded next to the test needles, and a drop of nitric acid is daubed onto all the test streaks. The base metal in the streaks vaporizes, leaving only the gold. The colour of the several streaks on the stone will vary, depending on the gold content remaining on the stone. This test is no good for heavy gold filled because you KNOW the surface is gold, but you want to know whether there is base metal beneath the gold.