A: Vertical Rotation pivot
B: Lock for 90-degree setting
C: Camera-mount screw (fits into camera base plate)
D: Alternate Camera-mount screw hole

This is set up for the Canon 50D & Sigma 17-70mm OS lens. The "C" mount is for 17mm, the "D" mount for 70mm. The "C" mount also works for the Sigma 10-20mm lens at 10mm.

This is for vertical mounting of the camera.