What is "privy mark"

A privy mark was originally a small mark or differentiation in the design of a coin for the purpose of identifying the mint, moneyer, or some other aspect of the coin's production or origin, for control purposes.

While the privy marked coins have been regarded as a design and marketing feature, some of these coins sell for huge premiums. This is the case for Canadian Silver Privy Marked Maple Leafs, which were introduced in 1998, and these sales can be observed on eBay.