Well, as I vaguely surmised, the bolt in the lower alternator bracket
was broken off--deep inside the channel. I have therefore given up and
made an appointment at the shop to have this extracted.

I mention this only as a follow-up to the many earlier posts, and to
keep the members who were kind enough to offer help posted.

The moral to the story is: If your alternator appears to be missing an
attach bolt, then the bolt is probably sheared off. This is a 19mm
bolt, and therefore, I did not even consider attempting an "easy out"
as I could not get at the shaft to drill it accurately. The shop will
probably remove the entire bracket in order to do this, but I was not
set up to do that at his time.

Thanks again, all!