Ray,
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
Dan R.
2004 FDTS40