If you unzip the entire data1.pak, which contains all the game level information, you'll get an assortment of files (some compiled, some not) that tell you what's in what level.
Lo and behold, you'll find it - so it's 'there', the question is, where in the game? You can check this yourself by unzipping data0.pak, then opening data/collectables.scr in notepad or something similar, and finally doing a search for 'Craftplan_Shotgunammo'. So it turns out, the Shotgun Ammo Mod is in the game - or at least, in the code.
