Noctua heatsinks ship with a second clip to enable you to add a second fan in a push-pull.
I bought a pre-built PC with this cooler, but the vendor didn't supply the clip to enable a second fan (and they don't have one for me). This clip only comes with the HSF itself, not with the fan.
Long shot...but does anyone out there have the Noctua NH-9 series HSF and not need the secondary clip they received?