That is a water filter and DCA dispencer. I think these filters are overlooked when changing filters. The Fleetguard WF2071 with DCA 4 units is the one's I have used on my 2001, ISC 350 and 2004, ISL 400 coach.
Here is a great link which tells all about the different filters and how many DCA units each has. Also info on antifreeze. http://www.cumminsfiltration.com/pdf...LI33021-GB.pdf
Also here's a great tip if your going to change it yourself. Right above the filter on the filter housing is a flat nub you can put a wrench on and turn 90 degrees. Which shuts off the coolant to the filter. Cuts down on the mess. But be sure to remember to TURN IT BACK ON