This episode is based on the first G. K. Chesterton (short) story by the same name of this episode "The Blue Cross".
The National Service Acts document calling for the services of Robert Flynn are dated for "Thursday 8 Oct. 1953". As he has already said that he is "sailing tomorrow" it is easy to date this episode exactly to Wednesday 7 October 1953.
Although with a slightly different ending, this episode follows the same basic story line as The Detective (1954) with Alec Guinness in the title role.