This is the camshaft end cap for the KV6 engine, commonly changed when the cambelts are serviced.
These are prone to falling off if fitted incorrect - please take care when fitting to make sure the sealing surfaces are clean and dry, and that the cap is fitted centrally.
These are made by the OE supplier to MG Rover, and are top quality.
Supplied and priced in pairs - if you need one, please see our other listings.