More than four dozen state lawmakers have penned a letter to Mastercard and American Express, urging the New York-based credit card companies to support a measure that could make it easier to spot suspicious purchases at gun stores. Credit card companies track spending based on the type of retailer where a card is used — though they do not track the individual items that are purchased. Each purchase is tagged with what’s called a merchant category code, with the master list set by an international standards body. Some categories are fairly broad, such as those for grocery stores and airlines,...