Gitzo GS5000 3/8 inch to 1/4 inch Steel Thread Adapter

Half the mounting problems on a set are one thread size out. This is the steel bushing that fixes them: it converts a 3/8 inch stud or plate to 1/4 inch, so a head with a 3/8 inch attachment goes onto a 1/4 inch tripod, or a 3/8 inch quick release plate takes a 1/4 inch camera screw.

It is steel rather than aluminium, which is what you want in a part that carries the whole load through a few millimetres of thread and gets wound in and out repeatedly. Two grams, and the reason to buy several rather than one is that the one you own is always in the other bag.