More information on Swivel Shackles:
Ronstan and RWO stainless steel ball bearing loaded swivels. Ball bearing swivels have lower friction and higher working loads than non ball bearing versions. Used in conjunction with blocks and rigging systems to provide articulation and rotation. Although not suitable for high speed rotating applications, they are ideal for vangs and furling systems. Made of 316 grade Stainless Steel.
Also in this category are swivels to convert your Harken fixed head blocks to swivel heads.