What is the 992 mark on silver?

Answer 1

It represents the metal's "fineness"; i.e. the fraction of the metal that's pure silver. An item marked 992 means that 99.2% of it is silver, and just 0.8% is another metal such as copper that's been alloyed with it for hardness.

Sterling silver is 925 fine, or 92.5% silver, with 7.5% of another metal.

Old US silver dimes, quarters, halves, and dollars (1964 and earlier) were made of "US coin silver" which was 900 fine. The other 10% was copper.