Thanks, Bruce.
That's exactly what I found except that, with a bit of imagination, I can see a dime-sized area on the upper part of the inner half that appears to be sort of flattened. The opposite "sunken" spot is quite distinct, but recessed next to the seam and raised at the outer side. This is the caliper that was mounted upside down and I found impossible to bleed. The odd thing is that, before I started to overhaul the brake system, that caliper was working -- not well, but it was doing at least half its job. So, someone must have found a way to bleed the bugger before I got to it.
Bob S.