"Hole" in my fan housing/timing cover?

[Abstemious]

Hey all, 86 300E, doing timing cover (hope that's where the oil's coming from), WP, pulleys, adjuster etc and noticed this opening in my timing cover. Can't imagine this is supposed to be left open, and what function would it serve? Can't find any photos of this.... probably will have to carve a plug? Waiting for info on this to reassemble the engine. Thanks