The M664BK is a black PP plastic 24-400 ribbed skirt child-resistant dropper assembly with rubber bulb and 108 mm glass pipette (for 4 oz glass dropper bottles). Dropper assemblies are composed of a number of parts: the cap, the bulb and the pipette (tube). The overall length of the dropper assembly is 126 mm (4.97 inches). The pipette length is 108 mm (4.25 inches). Dropper assemblies are used with liquids that require precision and intricacy when using the product. This closure has been equipped with a child-resistant feature. Polypropylene (PP) is marked with the resin identification code of 5. PP is less flexible than LDPE, somewhat stiffer than other plastics, reasonably economical, and can be translucent, opaque, or of any color. PP has very good resistance to fatigue, and has an extremely high melting point (320 degrees Fahrenheit or 160 degrees Celsius). This closure fits a container with a neck finish. The first number refers to the diameter of the container's opening (in mm). The second number refers to the thread/style/depth of the closure's skirt.