A very common map accessory, the bleistiftverlängerer (or pencil extender) allows you to more easily use every last bit of your pencil. It allows you to better hold even tiny stubs of pencils. I have seen period examples of these in original writing kits and finally found a modern made version. This is an extremely practical tool for your map case especially when you consider how hard it can be to get new pencils delivered to the front lines. It is quite simple to use, just inset the pencil and move the small ring band up to pinch the steel sleeve down to grip the pencil.
This is a newer made reproduction. The design is very classic and has not really changed in over 100 years. The handle is stamped D.R.P. ang. which is the German period equivalent of patent pending.