Comments about Nikon Nikon MS-40 AA Battery Tray f/MB-40:
Even though the MB-40 comes with one of these, if you power your gear primarily with AA (LR6) batteries, it's handy to have an extra MS-40 loaded with fresh cells in your kit. Simply swapping trays is much better than rushing to pull eight dead cells out of the tray and fumbling to insert fresh ones while watching the perfect shot pass you by as your light fades.
Though it's intended for use with the MB-40, it seems to work in the MB-D10 grip as well. Or it did for me when I accidently inserted it in mine. Your milage may vary.