The Lumicon 2" Drop-in Extension Tube (1.6" Push Back) provides 1.6" (~41mm) of extension to the optical path.  The front (where light enters) has a standard 2" outer diameter barrel, and the rear accepts 2" accessories which are secured with a pair of set screws.  This device is often required to correct the distance of the focal plane of a recording device when the telescope's eyepiece has removed.