In the novel Death in the Clouds, CL 52 is the code name given by Madame Giselle to one of her “clients” in her small black notebook. CL 52 is a married English peeress.
Hercule Poirot and Monsieur Fournier suspected that CL 52 might be Cicely, Countess of Horbury. Since she is a gambler, she could have borrowed money from Madame Giselle.