Neale Rabensburg has solved some puzzles about the Brill with his knowledge; and here he has illustrated for those of us working on The Book that Brill welts were hand-stitched, which explains how Newton Rabensburg was able to build them with two and three thick welts inside!
rabensburg exhibit 2018 (5).jpg this is Neale's image, of a holster left incomplete at N.J.'s death in '61. Notice the 'fishook' return stitching at the mouth of the welt, and that the welt itself was to be trimmed off later (today such welts are cut exact size, glued in place, and machine stitched)(but two welts is generally considered maximum)(Myres and even Bianchi and Safariland once produced double-welt holsters that were machine stitched).
boykinlp (3).jpg this is a boykinlp image of an N.J. We perhaps will never know why the welts themselves are not simply two individual continuous strips, but instead are a complex layering of skived welts that are interleaved not once but twice within the 'stack'.