This is an original overflow tank, removed from a 1977 MGB. It is also correct for 1968-74 1275 MG Midget and Austin-Healey Sprite and works as a "replacement" for the no-longer-supplied tank as used on 1975-1979 Midgets.
This one is in very good shape, a little grubby but no dents or holes. The cap is included. A new one lists for $100 and we don't know where they manufactured, or the quality. We do know this one was made in England a long time ago and is a nice product.