From the Manufacturer
This is the Axial 23T Aluminum Servo Horn for KO and JR Servos.
Hard anodized aluminum construction Compression mount, two screws
install on either side of arm and tighten center hole around servo
Axial HD Aluminum 6061 Servo Horn 23T fits KO and JR Servos, AX10, SCX10, Wraith. # AX30834