An association may invest its funds in the bonds of any local improvement district of any city of this state (except bonds issued for an improvement consisting of grading only), the ultimate payment of which is guaranteed by the municipality under the provisions of guaranty laws of this state: PROVIDED, That one-half of the lots in the district are improved with revenue producing houses or other improvements and that local improvement district bonds falling within the twenty-five percent, in amount of any issue, last callable for payment shall neither be acquired nor taken as security. The aggregate of the investments of an association in any issue of such bonds shall at no time exceed three percent of the amount of its savings accounts, and it may not have invested, at any one time, more than one hundred thousand dollars in the bonds of any such district.