clamp on sight for vent rib

Several years ago Cabela's had rifle sights that would clamp on to the vent rib of a shotgun. They were made of metal and not plastic. Anyone know where I can find such a thing now-a-days?

I have a 3901 Beretta I would like to get a slug barrel for but can't find one (don't know if they even made them) or put some kind of sights on it.