From Dienst & Hagedorn’s Latin American P-51D book.

This plate was found necessary in order to prevent propeller seal oil leak accumulations on the aircraft windscreen. The wind blast forced the oil onto the plate which caused oil spray to accumulate there. In this way, the windscreen was spared, thus allowing the pilot a good view forward. This minor modification is not often applied to civil Mustangs seen flying today.

T J