What is a factory vent rib on browning sweet 16?

A vent rib is a metal sighting plane attached to the top of the barrel from the receiver to the end of the barrel at the muzzle. They are usually a quarter inch wide and may have small grooves along the top. The venting means that it is raised above the barrel slightly to allow air between the rib and the barrel which helps the barrel to keep cool.

"Factory" simply means that it was originally installed at the plant that manufactured it. In trhis case either in Belgium or Japan. Many times aftermarket vent ribs are installed which hurts the value of a gun rather than increase the value. A factory installed vent rib increases the value vs a plain barrel.