A full pan is discouraged as it is supposed to slow ignition of the main charge. With a partial filling of the pan (keeping the powder below the touch hole for fast ignition from the flash), especially when hunting, the powder can slide to one side or the other of the pan (either covering up the touch hole, or placing the entire prime a centimeter away from the vent), and giving a smaller surface area for the shower of sparks to ignite. The "protrusion" fills the empty top of the pan with steel so the small amount of powder in the bottom of the pan can't all go to one side or the other of the pan, thereby insuring a larger surface area of powder is exposed to the sparks. I would think that a vent placed just above the top of the pan and blocked by the side of a flat bottomed frizzen with no "protrusion" and no "relief" filed on the underside for the vent, the "protrusion" might be superfluous if the pan is kept shallow. I don't recall an original with such a "protrusion", but it wouldn't surprise me if somebody shows us one. I don't like their looks and am not advocating their use, especially on American long rifles.