RAF WW2 Squadron Leader

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!

You’re thinking of those with the position (not rank) of Wing Leader.
https://en.wikipedia.org/wiki/Wing_leader

In December 1940, RAF Fighter Command began preparing for offensive fighter sweeps over France, using massed formations under the tactical command of wing leaders, also known as wing commanders (flying).[
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Wing leaders were permitted to use their initials on their aircraft in place of the usual squadron codes, to help identify them to their formation.

Spitfire PV270 is marked as Al Deere’s aircraft while he was Wing Leader at Biggin Hill.
PV270 takes off by Errol Cavit, on Flickr

Only Wing Commanders and ranks above were permitted to use personnel codes, not always their initials. Squadron Leaders sometimes had a particular letter that they preferred to use but were not allowed to use their initials.