It's a wretched stump of a pencil, villainously cut to an abominably bad point.

It is coloured dark red on the outside and was stamped with some name that began with C--O--Co-operative Stores, perhaps." "Now, my dear Jervis," Thorndyke protested, "don't begin by confusing speculation with fact.

The letters which remain are C--O.

Note that fact and find out what pencils there are which have inscriptions beginning with those letters.

I am not going to help you, because you can easily do this for yourself.