T2 extension tubes are intended for reaching the optimal back focus when using correctors like field flatteners or reducers. They are also needed for eyepiece projection. This variable T2 extension allows you to fine tune the distance by a fraction of a mm. The threads (male and female) are the international used T2 threads (M42x0,75)
Turn the connection part as long until the desired length is reached. Then, tighten the counter ring. Now you have a fixed T2 extension which is exactly the right length.