The penknife has the words NE Steel and Chief Insp. Sullivan says the penknife was made at a factory called North England Steel Works, but the painted sign of the factory has North England Iron Works.
When Rose Mallory is murdered, the two penknives form a T on the carpet and the cigarette smoke from the end away from the penknives. When Arthur Groome finds the body, the penknives are away from each other and the cigarette has been moved 180 degrees so the smoke is coming from the end towards the knives.