That's definately the vent, and it looks exactly as you have described on all the Benzs I have owned.

Access is always a problem. It was easy on my 123 chassis, extremely difficult on my 210 chassis, and is impossible on my 124 chassis wagon.

The only option may be to drop the differential to get access. As this seems a bit much work simply to clean the thing, don't think I'd do it unless there was a specific problem to address.