Notwithstanding the foregoing provisions, but subject to such requirements as the legislature shall impose by general or
special law, indebtedness contracted by any county, city, town, village or school
district and each portion thereof from time to time contracted for any object or purpose for which indebtedness may be contracted may also be financed by sinking fund bonds with a maximum maturity of fifty years, which shall be redeemed through annual contributions to sinking funds established by such county, city, town, village or school
district, provided, however, that each such annual contribution shall be at least equal to the amount required, if any, to enable the sinking fund to redeem, on the date of the contribution, the same amount of such indebtedness as would have been paid and then be payable if such indebtedness had been financed entirely by the issuance of serial bonds, except, if an issue of sinking fund bonds is combined for sale with an issue of serial bonds, for the same object or purpose, then the amount of each annual sinking fund contribution shall be at least equal to the amount required, if any, to enable the sinking fund to redeem, on the date of each such annual contribution, (i) the amount which would be required to be paid
annually if such indebtedness had been issued entirely as serial bonds, less (ii) the amount of indebtedness, if any, to be paid during such year on the portion of such indebtedness actually issued as serial bonds.