You are missing the long semi-hard rubber hose (about 3/4" diameter) that come up from the air pump at the lower right of the engine. It mates to the air pump filter which should be mounted on the cleaner where your 'unusual' opening is. The capped off hose could possibly be the line from the air pump - got a picture of the hose?
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.