This is theDubro Servo Arms Std Futaba (8).
For those requiring a really heavy duty servo arm, the Dubro "Super Strength" arms will fit the bill.
Black, fiber-reinforced composite plastic servo arms.
4 short, 2 long servo arms that extend in one direction from the servo center; one long arm that extends in two opposing directions from the servo center, and one 3-arm horn.
Short: Mounting holes at: 7, 10 and 12mm. Total length-16mm
Long: Mounting holes at: 8, 10, 13, 16, and 19mm Total length-22mm
Dual: Mounting holes at: 6, 10, 12, 16, and 18mm Total length-22mm
3-arm: Mounting holes at: 7, 10, 13, 16, and 19mm Total length-22mm
All measurements are from the center of the arm. The dual and 3-arm are only from one side to center. Actual overall length for both: 44mm
Fits the FUTABA servos with splined output shafts