Bill,
Looks like cheap steel to me. Weltmeister does not have a reputation for quality, even though their stuff does look pretty in the box.
For a good guide to fastener breakage, there was a long thread on Pelican recently started by "Craig911" about an ERP bump steer bolt that had sheared on his track car (the bolt was improperly installed). There was lots of good discussion/pictures/etc about bolt breakage.
I have the 22/21mm factory '86 Carrera sways on my car (with reinforced rear mounts). I'm going to get the Tarret adjustable rear drop links so I can set them up with no preload, and sometime in the future when I'm rich I'll get the Smart Racing sway bars, which are the simply the best there is (and expensive too).
I'd get on the horn with Weltmeister and request a new eccentric bolt.
