This is because the CS:S weapon models start out on the left, so the game flips them. Of course custom models will therefore flip over to the left.
The easiest way to fix this is with a small edit to the code.
In \garrysmod\gamemodes\terrortown\entities\weapons\weapon_tttbase\
At around line 80:
SWEP.ViewModelFlip = true
Change true to false.
This will result in a new lua cache, so if you use fastdl you'll have to update that.