Another item that has been lost to time on most MGBs. It is a seal that is attached to the rear edge of the bonnet that reduces the risk of engine fumes entering the air intake in front of the windscreen.
Only costs a couple of quid and easy to fit. Got mine from the MGOC (UK). It’s only a piece of self adhesive 20×10 foam rubber strip long enough togo along the width of the bonnet.