A will may refer to a written statement or list to dispose of items of tangible personal property not otherwise specifically disposed of by the will, other than money, evidences of debt, documents of title, securities, and properties used in trade or business. To be admissible under this section as evidence of intended disposition, the writing either must be in the handwriting of the testator or be signed by the testator, and must describe the items with reasonable certainty. The writing may be referred to as one to be in existence at the time of the testator’s death; it may be prepared before or after the execution of the will; and it may be altered by the testator after its preparation.