The Series D Bonds shall be serial coupon bonds. Said bonds and the interest thereon shall be payable at such place or places as may be designated by the State of Oklahoma Building Bonds Commission. The Series D Bonds shall mature on January 15 in the years and amounts, respectively, as follows:

The Series D Bonds shall bear interest, payable semiannually on January 15 and July 15 of each year, shall be in such denomination or denominations, shall be in such form either coupon or registered, shall carry such registration or conversion privileges, shall be executed in such manner, shall be payable in such medium of payment at such place or places, as shall be provided by resolution or resolutions to be adopted by the State of Oklahoma Building Bonds Commission; provided, that all such bonds maturing on and after ten (10) years from their dates may, at the discretion of the Commission, be made subject to call and redemption, in inverse order of bond numbers, at par or with premium, and accrued interest, with the detailed provisions for such calling and redemption thereof and the amount of the premium, if any, to be fixed by the Commission in the resolution or resolutions for the issuance of such bonds. Until such time as the sinking fund shall be sufficient to retire all outstanding State of Oklahoma Building Bonds of 1968, including interest thereon, there shall be paid into the sinking fund during each fiscal year from the sources hereinabove pledged such amounts as shall be necessary to pay the interest and principal as they become due. All bonds issued pursuant to this act shall have all the qualities and incidents of negotiable paper, and neither said bonds nor the interest earned thereon shall be subject to taxation by the State of Oklahoma or any county, municipality or political subdivision thereof.