I Googled the Mercedes part number on it, 0005410063 and the only link of any substance led me back to this forum.
It appears to be a part that is put on batteries made by Mercedes, I could find no trace of it on the Varta web site.
My vent trap was a little loose so I carefully forced a 11/32" drill end, not the drilling end, into the hole and warm it carefully with a heat gun until it got soft. I then cooled it under a tap and the part had grown enough to fit tightly in the battery. They probably get loose and fall out or are easily knocked out.
Failing to get one I would use the vent kit mentioned by ala_xfire, we have seen the damage done by spilled acid and it is not nice. The vent kit would be better in major spills, cars with batteries in the trunk are vented to the exterior of the car.