The US social security income is still reportable as taxable income on the F.1040 even though the income as described in Article 18(1)(b) is excepted from the saving clause under Article 1(5)(a).  Article 24(3) is the mechanism that is employed to eliminate US tax due on the otherwise taxable US social security income.  In other words, it should be taken care of through FTC on F.1116 and treaty disclosure would be required.