The letters are followed by a number representing the agency as a whole, or a subordinate office. FAA Airworthiness Directive Biweekly Listing The letters and numbers up to the colon constitute the class stem. The SuDocs number shows that this publication was issued by the Transportation Department, Federal Aviation Administration, and that it is the 19th Airworthiness directive issued in 1991. All numbers are whole numbers. There are no decimal points, only periods.
Nothing comes before something.
Letters are shelved before numbers.