Mich. Comp. Laws § 700.2513
Whether or not the provisions relating to a holographic will apply, 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. To be admissible under this section as evidence of the intended disposition, the writing must be either in the testator's handwriting or signed by the testator at the end, and must describe the items and the devisees 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; it may be altered by the testator after its preparation; and it may be a writing that has no significance apart from its effect on the dispositions made by the will.
MCL 700.2513