The International Silver Co. C306 mark appears on silver items produced by the International Silver Company, a major American manufacturer known for both sterling and silverplate products. The mark includes the company name and the model or pattern number “C306,” which helps identify the specific design or item within their catalog. While many pieces bearing this type of mark are silverplated, confirmation of silver content requires a separate “Sterling” stamp. The C306 designation is useful for matching items, verifying authenticity, and tracing production history.