A chalk outline of the body on the bathroom floor. TV and movie writers love the chalk outline. However, in the real world the outline is only used if a body must be moved before the crime scene examination is complete. Prosecutors hate the 'chalk fairy', that never identified cop who can't resist tampering with a crime scene and thus calling into question any physical evidence present.
The trivia item below may give away important plot points.
Dot finds a flat round case ( the shape of a contemporary diaphragm case) under Phrynie's bed which Phrynie says is a marvelous device invented by Marie Stopes. Stopes opened a family planning clinic in London in 1921 where she distributed her 'ProRace' brand of cervical caps. Those rubber devices were taller than they were wide, about 2 inches by 1 1/2 inches. Rubber cervical caps were invented by Friedrich Wilde in 1834.