This cover that sits under the oil cooler and fills the gap between the rad and the front plates guides air through the oil cooler then on towards the radiator fan. For some reason this is missing on my motor so now on the look out at auto jumbles for the part.
May be tricky to find as only used on the early rubber bumper cars 1974-75-76 before the rad was moved forward.
Found a picture of how and where it was fitted.
MGB tips mods and maintenance 