It's a trained ear thing along with the marks. There are two distinct sounds maybe thud isn't accurate. But the cast is more of a "dead note' or using musical terms the forged crank will "sustain" or ring longer than a cast which wont ring long at all.
As far as being good steel ,it was .These cars are from a time when Amerca was truly great, steel and cars and parts were made here or in Canada across the great lakes from Detroit. This is one reason why I love these old cars...:)
