So small and yet so indispensable!
Transformation thread with 3/8'' external thread that converts down to 1/4'' internal thread. 1/4'' thread is sometimes also called standard camera thread/screw and is the measurement that, for example, Manfrotto's quick-release plates and camera screws have. Fits as standard in e.g. Zeiss binoculars upon delivery, but sometimes breaks and can then be purchased as a spare here. Some binoculars are equipped with both dimensions threaded into the base, but only the larger quick-release plates are supplied with both screw dimensions. The transformation thread is also useful when, for example, a Manfrotto video head is to be mounted on certain tripods of other manufacturers, because Manfrotto as standard has a 3/8'' spigot on its tripods and a correspondingly threaded opening on its video heads.