Thanks. I have the feeling that something is unusual. I used to ride an old BMW motorcycle and enough of the problems I had due to old age and use - one of the last conditions they usually covered in the shop manual. This kind of reminds me of that except that the saddle is 9 years old and it's had a pretty easy life compared to some of the other Brooks saddles I've seen.
I'll measure the bolt before I try and put it back in. The rear edge of the adjuster doesn't even come through the front portion of the shackle, it seems much shorter than the ones in the link you posted. But the threaded end is smooth and doesn't appear to have broken off either. The threads on the tension bolt were inside the adjuster, I don't see how they could have broken off like that.
If I can't get this fixed somehow I'd like to ship things to you and let you take a look at it - I'd love to hear what's wrong and how things got that way.