The director of the department of administration in any municipality having a population of more than 300,000 which, prior to the effective date of this amendatory and supplementary act, had adopted the form of government designated as "Mayor-Council Plan C" provided for in article 5 of the act of which this act is a supplement, may appoint and may remove, with the approval of the mayor, an assistant business administrator of his department who shall serve in the unclassified service of the civil service of the city and shall receive such salary as shall be fixed by ordinance.
No municipality shall adopt the provisions of this section on or after the effective date of this amendatory and supplementary act.