Antoni’s statement is correct. The Wikipedia is, but only as far as it goes. The privilege went with the rank not position. For example, Wg Cdrs commanding an OTU, or in a staff job, often displayed their initials. There was some ‘creep’; Sqn Ldrs J.A.Storrar’s YT-JAS, Don Taylor’s JE-DT and Percy Beake’s Fighter Leader School Tempest PH-B come to mind. No doubt there were others!