You will need the IR-10, the C-1, and two of the 274300 o-rings to replace the ones between the cap and the sump.
IR-10
http://www.waterfilters.net/Waterpik-IR-10-Under-Sink-Water-Filter_p_0-375.html
The orings can be purchased here as well.
C-1
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/C-1.htm
The C-1 can be replaced with the IR-70E which filters more contaminants than the C-1.
http://www.waterfilters.net/Waterpik-IR-70E-Under-Sink-Water-Filter_p_0-379.html