It's a commonly known as a "Clothespin". Really! A little wooden clothespin to attach colored gels, diffusion to the barn doors of a light. The story goes that a key grip had purchased a couple bags for this purpose and was refused reimbursement from accounting because it was not considered equipment. So he re-billed for C-47s', being 47 were used and "C" for "clothespin". This was apparently in the fifties, but who really knows, it's just the story that gets past around from crew to crew.
Look up Grip Terms there are some good ones.
That's what I mean by the way, if someone didn't know what it was, you may be willing to pay $30 for these must need movie equipment to hold your diffusion in place. But you're going to be pissed when you get a bag of clothespins.
Dude, don't blow it. When someone asks what a C-47 is, you tell them it is a gel retention fastener!!!!! Are you trying to put me out of a job????
As an aside, wooden clothes pins are getting harder and harder to find. Metal clips get way to hot and the plastic ones melt. The future will be interesting.