The Commonwealth hereby pledges to and agrees with any person, firm or corporation or Federal agency subscribing to or acquiring any bonds issued by the city pursuant to the provisions of this act, that such bonds, their transfer and the income therefrom including any gains made on the sale thereof (other than underwriting profits in a distribution thereof) shall at all times be free from taxation within and by the Commonwealth of Pennsylvania, but this exemption shall not extend to underwriting profits or to gift, succession or inheritance taxes or any other taxes not levied directly on the bonds, the receipt of the income therefrom, or the realization of gains on the sale thereof.
53 P.S. § 16999.17