It is Steel No. 45, not stainless steel, it is a kind of iron, but it is harder than ordinary iron.
The coupling length is 22mm
The rigid coupling is 90 degree fastening design and is fastened by four machine screws.
The following 3 kinds of couplings are 16mm in diameter and 22mm in length.
6-10, one side aperture 6mm, the other end aperture 10mm; (side hole M4 machine screw)
8-10, one side aperture 8mm, the other end aperture 10mm; (side hole M4 machine screw)
10-10, one side aperture 10mm, the other end aperture 10mm; (side hole M4 machine screw)