Mich. Comp. Laws § 38.851
In order to extend to employees of the state of Michigan and its political subdivisions and to the dependents and survivors of such employees, the provisions of old-age and survivors insurance system embodied in the social security act, it is hereby declared to be agreeable to the state, subject to the limitations of this act, that such steps be taken as to provide such coverage to employees of the state and its political subdivisions, on as broad a basis as permitted by title II of the social security act.
In the event the positions of any coverage group covered by any public retirement system shall become covered under old-age and survivors insurance, then the aggregate benefits, old-age and survivors insurance and those of the public retirement system combined, for such public employees in positions covered by a retirement system on January 1, 1955, and for those receiving or entitled to receive periodic benefits from such retirement systems on January 1, 1955, shall not be less than the benefits provided by their respective retirement acts, charters, ordinances, or other official instruments of coverage in effect on January 1, 1955.
MCL 38.851