Section 19. The commission and all its revenues, income and real and personal property shall be exempt from taxation and from betterments and special assessments and the commission shall not be required to pay any tax, excise or assessment to or for the commonwealth or any of its political subdivisions; provided, however, that the commission is authorized to enter into agreements to make annual payments in lieu of taxes with respect to property of the commission located outside the municipality upon which local real estate taxes are being paid as of the effective date of acceptance of this chapter. Bonds issued by the commission and their transfer and the income therefrom, including any profit made on the sale thereof, shall at all times be exempt from taxation within the commonwealth.