N.Y.C. Admin. Code § 20-303: § 20-303 Keys to be marked.

All keys made by a licensed locksmith shall be stamped with his or her name and address or with the license number issued to him or her by the commissioner. All master keys in addition shall bear the word "master", and duplicates shall be made only upon the signed order of the owner of the locks which such keys are designed to operate or his or her authorized agent. Master keys shall be made only by a licensed locksmith.