After October 13, 1967, a political subdivision shall not commence coverage of its employees who are neither policemen nor firemen under another plan similar in purpose to this system, other than under this system, except the federal Social Security Old Age, Survivors, and Disability Insurance Program, as amended; except that, any political corporation or subdivision of this state, now having or which may hereafter have an assessed valuation of one hundred million dollars or more, which does not now have a pension system for its officers and employees adopted pursuant to state law, may provide by proper legislative action of its governing body for the pensioning of its officers and employees and the widows and minor children of deceased officers and employees under a plan separate and apart from that provided in sections 70.600 to 70.670 and appropriate and utilize its revenues and other available funds for such purposes, and except that the board of hospital trustees of any hospital which is owned by any political corporation or subdivision of this state, may provide for the pensioning of its employees and the widows and minor children of deceased employees under a plan separate and apart from that provided in sections 70.600 to 70.670, and utilize its revenues and other funds for such purposes.