Yep, It's the brass shell.
I have a Yamaha (beaded|) brass shell 6.5 X 14 concert snare. its so responsive at any playing volume. You can literally get any sound you want out of it, except the warm woody tones of course.
The older Ludwig & Ludwig models with the hexagonally beaded brass shells sound very robust due to the NOB shells. They are far too loud for most playing situations today except rock because they were designed (like the BB) to cut through the bands of the swing era. It was literally Brass (horns) on Brass (snare). The nice thing about the 5.5" BB is that it still fits in smaller ensembles but still cuts like a knife through every instrument but at a lower volume than the 6.5".
Not so sure that the 20's version is superior sound wise than the newer Black Beauty's as the same beaded bell brass shell is produced today that Ludwig has produced since the start. With the addition of triple flanged hoops on 70's forward versions, they may actually sound better then the 20's BB.
cheers!
-kellyj