The Nikon BL-3 Battery Chamber Cover is used with the MB-40 or MB-D10 multi-power battery pack with the EN-EL4 and EN-EL4a batteries. The MB-D10 battery pack is for use with the Nikon D700, D300, and D300S digital cameras. The MB-40 is for use with the F6 film camera.
I had gotten a Nikon MB D10 battery grip but it came without the BL-3 so I ordered one from B&H Photo. It arrived in no time at all and works very well with my ENEL4 batteries so I am very happy with the purchase.
I have purchased a MB-D10 Chamber cover for every battery I have so that the change out of the batteries is as quick as possible not losing the chance for a great action shot when I am shooting wildlife.