A mutual savings bank may invest not to exceed fifteen percent of its funds in the bonds or warrants of any local improvement district of any city or town of this state (except bonds or warrants issued for an improvement consisting of grading only), unless the total indebtedness of the district after the completion of the improvement for which the bonds or warrants are issued, plus the amount of all other assessments of a local or special nature against the land assessed or liable to be assessed to pay the bonds, exceed fifty percent of the value of the benefited property, exclusive of improvements, at the time the bonds or warrants are purchased or taken by the bank, according to the actual valuation last placed upon the property for general taxation.

Before any such bonds or warrants are purchased or taken as security the condition of the district's affairs shall be ascertained and the property of the district examined by at least two members of the board of investment who shall report in writing their findings and recommendations; and no bonds or warrants shall be taken unless such report is favorable, nor unless the executive committee of the board of trustees after careful investigation is satisfied of the validity of the bonds or warrants and of the validity and sufficiency of the assessment or other means provided for payment thereof: PROVIDED, That, excepting bonds issued by local improvement districts in cities of the first or second class, for improvements ordered after June 7, 1927, no local improvement district bonds falling within the twenty-five percent in amount of any issue last callable for payment, shall be acquired or taken as security.