My Beretta is a hinge gun, so, like .50cal Packer, I'm not a 391 guy. The "hole" in question is the exhaust port. IINM, controlling the gas works is done more with vale spring pressure than the size of the exhaust hole. I concur with those who've commented that the hole in question seems to have been modified by the gunsmith's apprentice.
With gas guns, the gas must go somewhere after it's completed its task. With the Remington 1100 there are notched vents in the top of fore-wood. You can see exhaust gunk on both sides of the barrel after an 1100 has been exercised. This isn't a problem unless you happen to have a shooting style that places your finger/s over one of the vents. Back in the day, it wasn't unheard of to rework an R-1100's fore-end to redirect the exhaust.
With Berettas, there is an exhaust grill assembly at the tip of the fore-end. If you place your hand under that grill, you might get a mitt full of warm gas. By enlarging the exhaust port's size, you'll reduce the exit gas pressure and possibly eliminate/reduce the hot hand problem. Or, perhaps it helps to keep the gas works cleaner (Do those exhaust vents get gunked-up after prolonged shooting?). My 2¢ worth, YMMV