(a) Admission of equipment and materials. Equipment for use in construction of bridges or tunnels between the United States and Canada or Mexico shall be admitted without entry or the payment of duty. Materials for such use shall be admitted without entry or payment of duty only for installation in the bridge or tunnel proper, and not in the approaches on land at the United States end of such bridge or tunnel.

(b) Customs supervision. All articles admitted under paragraph (a) of this section shall be subject to Customs supervision at the expense of the builder until installed, entered, or exported.