This is a Toe Gauge Adapter.
Used with Toigo Racing's custom Carbon Fiber Sleeved Axles.
Brands it works with are:
Proline Toe Gauge System
Performance MFG Toe Gauge System
1. The part with the blue end slides into the carbon fiber sleeve
2. The threaded part threads into the blue end
3. Connect your mirror to the part you threaded into the blue end