The legislative body may fix a date, not more than two years from the date of issuance, for the earliest maturity of each issue or series of bonds and in the case of bonds issued for the acquisition, construction, or completion of revenue-producing public works may fix a date not more than 10 years from the date of issuance for the earliest maturity of each issue or series of bonds. Beginning with the date of the earliest maturity of each issue or series, not less than one-fortieth of the indebtedness of such issue or series shall be paid every year; provided, however, the bonds of any issue or series irrespective of the purpose for which the same are to be issued may be made to mature and become payable in approximately equal total annual installments of interest and principal, during the term of the bonds computed from the first year in which any part of the principal shall mature to the date of final maturity which annual installments may vary one from the other in amounts not exceeding in any year more than 5 percent of the total principal amount of the bonds of such issue or of the series thereof then proposed to be issued. The final maturity date shall not exceed 40 years from the time of incurring the indebtedness evidenced by each issue or series.