Canon CB3-3972-000 is a cap for the extension terminal, listed in the Canon EOS 7D parts catalogue. It is a specific camera component identified by its full part number, making it easier to distinguish from other covers and caps in the catalogue.
The parts list names its function as an extension terminal cap, and the accompanying diagram marks the same part number. The catalogue lists one piece for the EOS 7D. This description is limited to the details confirmed in that parts catalogue.