The GSO Camera Extension Rings Kit (M42) consists of 5 elements.
4x Extension rings M42 - M42 in lengths 5mm, 10mm, 20mm and 30mm, which can be stacked in various combinations.
1x Variable Locking Ring M42, which can be used with any of the above extenders, to achieve a fine adjustment of spacing in millimeters of 1-4mm or 1-5mm depending on the extender ring connected with it. 
With this extension ring kit several combinations can be made to reach any extension needed for your astrophotography setup.