Miss Marple finds a library copy of the Pelican edition of "To Define True Madness: Commonsense Psychiatry for Lay People" in Molly Kendall's room, with Date Due stamps ranging from 1941 to 1951; this book was first published by Penguin in hardback in 1953, and the Pelican edition was released in 1955.
There is a portrait of Queen Elizabeth II in the office of Napier, the Administrator however the cap badge of Inspector Weston has the royal cypher of King George VI.
Major Palgrave and Inspector Weston both refer to the Major as having been a member of the St Kitts (or St Christopher) and Nevis Police around 1948-51. St Kitts Nevis & Anguilla never had there own force until 1960, before then they were part of the Leeward Islands Police Force,
When Miss Marple finds the book under Molly's mattress it is due to be returned in February 1951 but the portrait of the Queen means the events are taking place after February 1952 when the Queen came to the throne.