Yes Joe, the Beretta 303 is a forerunner of the 391. They are excellent shotguns and a real pleasure to shoot. They were made for a fairly long time, so you should be able to find lots of accessories.
After the 303, I believe Beretta brought out the 390, which is an excellent shotgun but has a square back receiver which sold well in Europe but not here. So, they then brought out the 391, which has a round back receiver.
Good shooting.
Big Paulie