This is the retaining clip that holds the pins in the rear calipers. These are often twisted in odd shapes from people using a Phillips screwdriver improperly. If yours is messed up, don't reuse it. If it comes loose, the pins could potentially walk out and into the spokes of your favorite wheels. Don't let that happen!
Please be aware that Nissan / Nismo / Infiniti pricing is subject to change.
