It shall be unlawful to keep for the purpose of sale, offer or expose for sale, or sell any sewing, basting, mending, darning, crochet, tatting, handknitting, or embroidery thread, put up or packaged in advance of sale on spools, tubes, cones, bobbins, or in balls, skeins, or other similar forms referred to in this chapter as units, for either wholesale or retail sale, unless each of the units is definitely, plainly, and conspicuously marked to show the net weight in terms of avoirdupois pounds and ounces, or the length in yards of the thread, subject, however, to all of the following provisions of this chapter.