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.
Won Nikon D700 in an auction, finding battery and parts was difficult. But thank you B&H!
If you have the MB-D10 Multi-Power Battery Grip for your Nikon D700, then you really must also have the BL-3 Chamber cover as it is necessary if you want to use the EN-EL4a battery. The EN-EL4a battery will increase you continuous shooting rate up to 8 frames/second on the D700 & also let you fill up a 32GB memory card or two without having to change the battery. This is a real plus, especially when doing serious wildlife photography as I do.