I added an extended mag weapon mod to my weathered 10mm pistol and the model file changed to a large exclamation mark.

I got the same problem, when i get back from work I'm going to see if a can make a patch... I'll post if I get it to work

one of the Bethesda workers saw a post on this yesterday said it was being looked in to

I checked it out, it is rather simple, the models for the modded weapon are not included in the DLC. If you look in the ClassicPack.esm on the weathered 10mm pistol's "Mod Info" tap the 7 models that are referenced, are not in the ClassicPack.BSA