The Nikon BF-3B Replacement Front Mount Cap for Nikkor AF-S Teleconverters replaces the BF-3A. It is a protective cap that goes on the front of your teleconverter. A tight seal, thanks to the bayonet lock, prevents liquids, dust and dirt from ruining your teleconverter. This cap is compatible with the TC-14E II, TC-17E II, TC-20E II and TC-20E III teleconverters.