What does 550 mean on silver salt and pepper shakers mean?

550 on silver salt and pepper shakers means that the silver is 99.9% pure. This is the highest standard of silver purity, and is often referred to as "fine silver." Silver with a purity of 99.9% or higher is considered to be "sterling" silver, and is the standard for most jewelry and silverware. The 550 mark is typically stamped on the bottom of silver items to indicate their purity.