Compiled Laws, Chapter 168. Michigan Election Law § 168.745

Sec. 745. Whenever at any election the ballot of any person who has been challenged as an unqualified voter and who has taken the oath provided by law in such case to be taken shall be received by the inspectors of election, said inspectors shall cause to be plainly endorsed on said ballot, with pencil, before depositing the same in the ballot box, the number corresponding to the number placed after such voter's name on the poll lists without opening the same: Provided, That in case a ballot shall be so folded, defaced, printed or prepared that such number cannot be legibly and permanently written on the back thereof, said inspectors shall refuse to accept such ballot.