This part doesn't appear in Bentley, unless it's a non-pictured part of the heat exchanger itself. 

Inside the heater box, sandwiched between the core and the blower manifold openings, is a piece of sheet metal that doesn't quite cover the whole face of the core . It has one row of small holes along one long side, and three rows of holes along the other long side. It obviously blocks large debris, but also restricts airflow to about 50% of the core.

What is it, and can I leave it out for greater heat exchange efficiency?