Technically, those are ejector shrouds, or ejector rod shrouds. It has become common to refer to them today as lugs, or under lugs, or under barrel lugs, but they are really shrouds.
This is an under barrel lug. Its purpose is to provide a forward latching position for the front end of the cylinder rod, particularly on a S&W revolver. Colts don't have them, they lack a way to latch the cylinder rod at the front end, the rod just hangs free. The purpose of a shroud is to protect the cylinder rod. A lug does not offer much protection to the cylinder rod.