The governing body of any municipality of the state of Kansas having a population of more than one hundred twenty thousand (120,000) and owning and operating a public utility or utilities is hereby empowered and may at its option enter into an agreement with its employees in said municipally owned public utility or utilities providing for the establishment of a retirement pension plan for the benefit of said employees, their wives and dependents: Provided, That in any municipality of over one hundred twenty thousand (120,000) population where the public utility or utilities are operated, managed, and controlled by a board of public utilities or any other managing board, however designated, such managing board instead of the governing body of the municipality shall be empowered to enter into the agreement and establish the retirement pension plan provided for herein.