It was shaped to clear a speedloader. A fair number of cops carried these longer M-19's and I had three. Very accurate and gives higher velocity than shorter .357's.
Gun writer Jeff Cooper felt that the six-inch M-19 was a fine trail gun, giving full .357 power with less weight than a M-27 or M-28. Of course, it'll also fire .38 ammo, to kill edible small game.