Nikon description says the BF-3A can be used as a body cap.
Nikon description of BF-3B says
"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." No description for use as body cap, though.
There is the BF-1A and BF-1B which are the real body caps, but there is no explanation of what is better/worse in using those vs BF-3A/-3B.
Maybe someone with long history with Nikon stuff could explain what the product descriptions from Nikon do not explain.
In doing some research,
- it seems the body cap does not fit as well when used as a teleconvertor cap. There is not sufficient room with the BF-1x to fit over protruding optics on a teleconvertor which protrudes.
- And the BF-1B seems to stay on the body better than the BF-1A. Nikon quietly increased the diameter of the lens mount flange on its AF SLRs, so the older BF-1 does not fit the AF SLRs.
- The BF-3 diameter is 56.32mm, while the BF-1A diameter is 57.28mm - 57.37mm.
It is all made more complex by the number of different teleconvertors from Nikon, and the several changes in teleconvertor cap compatabilty.