This swivel clamp is used to connect your 6mm eyesight post to your 1/4" articulating bracket or mounting post. Standard size is 1/4" so this is only used for European based ringsights, not our concentric! Use the 1/4 x1/4" Swivel Clamp for these. 

Fastened with an Allen bolt, this mount prevents your eyesight from ever moving. The old knuckle screw type sucks in comparison to this. If you happen to still have that type, seriously consider upgrading. It's so worth while.