In SR2, each mission would specify mission-specific assets to load in the cts file. For example, the Brotherhood mission bh02 has you going to the nuke plant. The bh02.lua would be the mission script and bh02.cts would have all the coordinate data, and the defined mission specific assets to load like the gieger counter weapon wesh. The cts would also have other unique things like animation, vehicles, and npcs. Anyway, there are no cts files in srtt that correspond to the missions. It is likely a binary file somewhere that contains all this data and what is loading the weapon meshes.
Most DLC files seem to be restricted in certain ways from being accessed by the main game in the common ways we normally use. All weapon based DLC meshes like the chum gun or bling shotgun are not part of the DLC archives at all. They are part of the main game files and is how they are being preloaded. Before you ask, I've already tried adding DLC meshes into the regular archives to see if they load, but the exe isn't doing anything with them if they are present there. My guess is that the game first loads/preloads all the regular assets, and is looking for certain file names. Then it loads/preloads DLC assets. How it loads the DLC is still a mystery though.