In the files section here, there's a pdf comparison of W&W bobbins that includes an Adler and a Jones Spool.? These dark metal bobbins have a different appearance although they seem identical in diameter and thickness to standard W&W bobbins.?
Speaking of Adlers, it might be possible that these are European after-market bobbins made for W&W or for copies of W&W's.? At any rate, it appears that these bobbins aren't common.? I may never know their source.? ? ?? ? ?
Roxy