Type of clew, also called an Easy Trim Clamp, as ropes may be tightened, but not loosened, without removing the load and opening the clamp via the wingnuts.
http://www.productionadvantageonline.com/Fly-System-Hardware/HR2840.aspx
http://www.productionadvantageonline.com/Fly-System-Hardware/HR2840.aspx