That metal tab is called a "Fear Rattler" and was pattented by Ruger. It let's you know if you're afraid of the gun and start shaking from fear. It won't allow the gun to fire until you man up and stop shaking.
Perfect for hunting late Fall any and all timber rattlers will exit your area knowing you're king of the hill. That said don't carry it during Spring mating season, LMAO...
Seriously, nice gun good luck with it. That rattling would bother me too.