Thought this was the air filter on Westerbeke W30

[Hoodsail]

I have a 1978 Westerbeke w-30 diesel (4 cylinder). Bought an air filter element to install, reviewed the technical manual and unbolted what i thought to be the metal "box" that contained the element. Nope...the rectangular part (see pics) that was attached to the top of the heat exchanger is one piece with a port for a 1/2" hose, that appears to feed into the crankcase. The underside (see pic) of this part has a thin film of oil on it and opens to what appears to be a manifold above the heat exchanger.
Questions - is this a crankcase breather? where is the air filter & element?
Per the tech manual (see pics) the air filter is where this part is bolted on...

Apologies for schematic & top of engine pic (part removed is behind the hi pressure (silver) cap) are up side down...no matter how i rotate these 2 pics they always appear upside down.
Any guidance would be much appreciated..
thnks
dave