The HP 840682-001 Hard Drive Hardware Kit is a service part associated with the hard-drive assembly in supported HP ProBook notebooks. It is listed separately from the drive itself, so it serves a different role from the storage device.
HP’s service guide lists this kit for the ProBook 640 G2, 645 G2, 650 G2 and 655 G2 series. It is intended for work involving the hard-drive assembly in these notebook families, such as replacing or servicing that assembly. The guide does not specify the kit’s individual contents, so this description does not assume which brackets, screws or other pieces are included.