Tax-free sales.

For provisions relating to the tax-free sales of articles referred to in section 4161(a) see: (a) Section 4221, relating to certain tax-free sales; (b) Section 4222, relating to registration; (c) Section 4223, pertaining to special rules relating to further manufacture; and (d) Section 4225, relating to exemption of articles manufactured or produced by Indians; and the regulations thereunder.