This servo is compatible with servo arms designed for M5 T21.
https://www.deeforce.net/product-page/cnc-servo-arm-for-s1855-e397-e3977

Dimensions: 0.31 x 0.93 x 0.87 in / 12 x 23 x 22mm
Weight: 0.51oz / 14.5g
Stall Torque (oz-in): 24.8@5.0v/ 28.8@6.0v/ 34.7@7.0v / 41.7@8.4v
Stall Torque (kg-cm): 1.8@5.0v/ 2.1@6.0v/ 2.5@7.0v/ 3.0@8.4v
Speed (sec/60deg) : 0.13@5.0v/ 0.11@6.0v/ 0.09@7.0v/ 0.08@8.4v
Motor Type: coreless
​​​​​​​Spline size M5-T21
System PWM
Gears Material: B/plastic, C/Metal, D/Met, E/Metal. F/Metal.
Designed to take up to 8.4V, but recommended voltage range is from 5-7V.
 
Important Notes on Using Our Servos
Our servos are programmed to operate with high precision and sensitivity.
Because of this, if there are any obstructions within the servo’s range of motion and it is unable to move as commanded by the transmitter, the servo may experience excessive load. This can result in damage to the internal circuit board due to overheating.
When installing the servo for throttle, aileron, or other control surfaces, please make sure to leave a small amount of free play between the servo travel range and the mechanical movement range. This helps prevent any binding or overloading of the servo.