That cover is called a suppressor housing and can be had at FastLane for ~$23.00. I would think it serves to keep dirt and moisture from the ignition wire ends at the distributor - a nice touch in my book. To tell if your car was ever fitted for one, take a look at the distributor cap - there should be three small rectangular protusions/recepticles, those are where the three legs of the housing attach.
'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.