How can you fix something by changing how it works beyond what was intended? The pistol is a unique, and no unique in the game allows for mods, so the fact that you could put a mod on the pistol was the problem, not the lack of models.
Then again, that design decision made no sense to begin with, unique weapons are in essence exactly like the regular ones, they only look different and have names, how does that prevent me from adding a silencer or scope? Nobody is dumb enough to design a weapon that is not compatible with any sort of modification in reality.