C47

Clothes pin you say? Not on set... During the production of a movie, commercial, music video etc., a spring-type clothespin is called a "CP 47", "C47", "47", "peg", "ammo", or "bullet". It is most useful on the set since lights used on film sets quickly become far too hot to touch; a wooden C47 is used to attach a color correction gel or diffusion to the barn doors on a light.[6] The wooden clothes-pins do not transmit heat very effectively, and therefore are safe to touch, even when attached to hot lights for a significant period of time. Plastic or metal clothes pins are not used as plastic would melt with the heat of the lights and metal would transfer the heat making the clothes-pin too hot to touch. People like gaffers, grips, electricians and production assistants may keep a collection of C47?s clipped to clothing or utility belt at all times.[7] Hence the nickname "bullet", as so many crew members clip a number of C47s to their utility belts, much like an old west gunslinger would carry extra bullets on his gun belt.
When a talent is in full makeup they some times can not drink from a cup so they drink from a straw. When the bottle or cup is too deep for the straw a C47 is clipped an inch from the top of the straw to keep the straw from falling into the drink.
The name "C47" may have come from an attempt to make it sound less mundane than a clothespin, or it may have come from the label on the bin used to store them in an early studio.[8] More commonly believed is that the name "C47" came to be the designation that the clothespins were given when printed on studio budgets to trick budget managers into approving the request for them. A "C74", "74C", or "A47" is a clothespin that has been taken apart, reversed, and put back together so that the small end comes together. This gives a tweezer-like tool, useful for a task such as pulling a scrim from a hot light.
All credit to Wikipedia for the description of the good ol' C47