found at DigiDirect
Dedicated to the Sigma TS-111 Tripod Socket for 105mm f/1.4 DG HSM Art Lens is a single locking knob collar which is used as a replacement that came originally with the lens. Once the collar is screwed and fixed tightly, it enables the lens to easily turn on axis in a horizontal or vertical position. In addition, the dovetail foot on the tripod socket is compatible with many of the Arca swiss type clamps.
Note:* This socket is compatible with the Arca Swiss screw knob clamp. The socket should be tightly screwed and fixed. It is not designed for lever type clamps. There are some lever clamps available that do not fix firmly and safely. Two Safety Stopper Screws (supplied) should always be fixed so that the Arca Swiss clamp will not fall off.