A C-47 is the film industry term for a wooden clothes pin which is used to attach a sheet of gel to the barn doors of a light. The term came about when a cameraman billed a major network in the 70’s for clothes pins. That network said “you can’t bill us for clothes pins” to which the cameraman replied “…but I used them on your shoot.” So he changed the name to C-47 and the expense item was approved.
A C-47 is always wooden, never plastic.