The difference between 14/15 and 19's is cylinder length. After a certain point S&W figured out they could fit .357 power into the K frame. After people started experiencing cracked forcing cones (at the thin spot on the 6 O'clock position) from heavy .357 use, S&W discontinued the .357 K frame and introduced the L frame, which did not have the thin spot, to allow full time use of .357. The K frame .357's will last almost forever under .38 loads and are fine with small amounts of .357's. Stick to 158 grains or heavier in the magnum loadings to prevent early erosion.
Recessed cylinders were discontinued to simplify manufacturing except on rim-fires, where they were kept as a safety measure. I really love the way a recessed cylinder looks though, it's a beautiful touch of a bygone age.