Extension Tube with a male and female M42x0.75 (T2) camera-accessory thread on each end.
It has an Easy-Grip knurled ring for easier loosening / tightening.
The spacer can be "stacked" with other T2 spacers and T2 variable spacers in the range to achieve the exact distance you require between camera and flattener.
Can be stacked with other spacers to get the correct spacing between your astrophotography camera and reducer or flattener optics.