This is the Hot Racing Blue Aluminum I Shaped Arm Servo Horn.

Anodized Blue Aluminum Spline adapters for universal application

One Aluminum double armed servo horn,
Three servo spline adapters
Two screw collar.

Standard sized servo

length: 1.7" (43mm) width: 0.47" (12mm)mcw

- 23 tooth (KO, Sanwa, JR, Airtronics, Spektrum)
- 24 tooth (Hitec)
- 25 tooth (Futaba, TRA, SAVOX, Tactic, some of the new Hitec)