Just a thought. I know folks have referred to the center rib on these drums as the bead. We shouldn't forget though that "bead" is a common welding term. When a welder runs a bead, he is running a weld that has some kind of length to it. On Ludwigs the center rib had two pieces brazed twice along the center rib. A welder might see that as running a “double bead” and that could have been the origin of Ludwigs use of the “double bead” term.

I'm sure our welder friends here on the forum could shed more light on this than I.

Those old "double bead" Ludwig shells have been demonstrated to have had 2 horizontal seams and 2 vertical seams per drum.

Here is a link to the 1918 Ludwig catalog that refers to a "rib" to strengthen the shell:

http://www.vintagedrumguide.com/images/ludwig_snaredrums/1918-ludwig-snare-drums-2.jpg