Weapons aren't unlocked even after completing all Bell Towers
This is a not-uncommon bug with Far Cry 4. Sometimes completing collectibles won't award the achievement or weapon it was intended to. In my playthrough I wasn't able to collect the achievement for acquiring all of Mohan's journals, despite otherwise 100%-ing the game. There isn't a workaround to this that I've found. Even uninstalling, reinstalling, and replaying the game didn't fix the bugged achievement for me - and I suspect it wouldn't unlock the weapon for you.
I'm sorry, but I don't think there's anything you can do.