This RDS (Rotary Drive System) adapter is 25mm long and fits servos with standard-size 6mm-25T ISO splines like ProModeler (to include sub-micro, micro, mini, and standard sizes) as well as compatible servos, e.g. from Savox, MKS, et al.
How do you use this thing? Common practice for using these is to glue in a short length of pultruded carbon fiber tubing (using JB Weld steel-filled epoxy), to form the basis of the linkage. From there the limits are your imagination!
Note1; we also offer PDRS12RDS-M5 for 5mm micro-spline servos like the 8mm thick DS115CLHV wing servo.