(a) As a prerequisite to the free entry of bolting cloth for milling purposes under subheading 5911.20.20, Harmonized Tariff Schedule of the United States, the cloth shall be indelibly marked from selvage to selvage at intervals of not more then 10.16 centimeters with “bolting cloth expressly for milling purposes” in block letters 7.62 centimeters in height. Bolting cloths composed of silk imported expressly for milling purposes shall be considered only such cloths as are suitable for and are used in the act or process of grading, screening, bolting, separating, classifying, or sifting dry materials, or dry materials mixed with water, if the water is merely a carrying medium.

(b) Bolting cloths not marked in the manner above indicated at the time of importation may be so marked by the importers in public stores under the supervision of customs officers.