It shall be the duty of every county, city, town, township, the board of education of any city, and of every school district, issuing bonds under this article,¹ and of the proper officers thereof, to create a sinking fund; and there shall be levied by the proper officers, annually, a sufficient tax therefor for the redemption of such bonds, which shall be collected as other taxes, and paid into the treasury as provided by law for other taxes, and shall remain as a specific fund for the redemption of said bonds; the amount of which sinking fund shall be as follows: In every instance in which bonds shall be issued under this article, for twenty (20) years or less, the quotient found by dividing the amount of the principal of such bonds by such number of years shall be the amount of sinking fund to be levied each year for the redemption of such bonds; but in every instance in which such bonds shall be issued for more than twenty (20) years, it shall not be necessary to create a sinking fund, or to levy a tax therefor, until the twentieth year prior to maturity of such bonds, at which time, and each year thereafter one-twentieth (1/20) of the principal amount of such bonds shall be levied as a sinking fund for the redemption of such bonds: Provided, that any county, city, town, township, the board of education of any city, or any school district, issuing bonds under this article, may buy in and cancel any such bonds whenever the same can be done at or below par: Provided, further, that such sinking fund, when not required for the payment or purchase of bonds, may be invested in bonds of the United States or of the State of Oklahoma, and in no other manner: And provided, further, that under the provisions of this article, the proper officers are authorized, if desirable, to issue installment bonds, running twenty-five (25) years, having coupons attached, representing the semiannual interest to become due thereon; and each coupon attached to any installment bond shall, after five (5) years from its date, represent one-fortieth (1/40) of its principal, which amount shall be shown by separate words and figures aside from the interest represented in the coupon; and each installment bond shall show upon its face that its principal is included in its coupons.