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"KM coin numbers refer to the numbering system used by the Standard Catalog of World Coins, originally written by Krause and Mishler. The catalogs are still published today, with each volume covering a century of coins, so there is a volume for each the 17th, 18th, 19th, 20th and 21st centuries." --GreatCollections.com
"Each type and denomination of coin from a country is assigned a KM number. Many collectors use the KM numbers as an easy way of keeping track of coins they want or have. By having the catalog and knowing the country name and KM number, collectors can look up the coin in the Standard Catalog of World Coins" - https://www.joelscoins.com/km.htm
From these explanations, I would assume that if you know the catalog number, you could add it to the listing as an item specific to help buyers more quickly locate coins they are seeking.
