Lacerta Variable Locking T-2 Extension 24mm - 34mm Optical Length
Variable locking T-2 extension tube (24mm -34mm variable optical length), consists of 3 parts.
The part with the longer thread (top left corner of the image) can be used as a variable legth extension on its own right between 8mm to 18mm when used together with the locking ring, depending on the depth of the female accepting part on your accessory, but even in worst case, you may use it as a 15mm-18mm extension, plus the longer part (top right corner of the image) can be also used separately as a fixed length 17mm T-2 extender.
There is also a slightly shorter version from TSOptics (20-30mm). See here: TSOptics Variable Locking T-2 Extension 20mm - 30mm Optical
It is a very useful accessory when it comes to special imaging setups. See below an example of a planetary imaging setup with a Imaging Flip Mirror, ZWO Filter Wheel and ZWO ASI120MM monochrome camera. With the help of a Variable Locking T2 Extension (there are two slightly different versions, one from Lacerta and another one from TS Optics) we could achieve parfocality. The eyepiece holder (that is part of the Imaging Flip Mirror) is also adjustable, so with these two adjastable items we can achieve various positions of the eyepiece thus parfocality can be achieved with eyepieces of various focal lengths. To see approximately the same field, you'd have to use an eyepiece with approximately 6mm focal length. Please note that this setup might not work for a Newtonian telescope due to the long back focus requirement.
Click on the above image to see it in full resolution. Image is for demonstration only. Flip mirror, eyepiece, camera, filter wheel and telescope are not included.