I already checked with original size of servo's bolt. (which are inside servo bag if you buy it online.)
[length is exclude bolt head)
Horn's axis x1 : ØM2.5 , length= 3.5mm
Shoulder mounting x2 : ØM2 , length= 8mm
So, if anyone already have a fancy anodized bolt or nut or washer from RC parts, you can add extra total length of bolt you need when add washer, and replace it on Nybble if you like.
Caution : Don't forget to check any clearance of parts.