is it located under the fitting that the inlet line connects to?
you may need to contact the injection pump mfg for that one, it may be part of the pump assy.
You could always contact the cat dealer and see if they have any parts notes about this filter if it does not show in the manual.
I know alot of those line filters are part of the banjo bolts but in this case i have a feeling from what i remember that this is part of the injection pump and not listed in the parts book.

I do not have direct access to my cat manuals at the moment but if you could post a serial number of this truck i may can do some checking and see if i can find it (if it is listed in another part of the book)