Got it Fixed! Couldn't find any obvious broken or misaligned parts, so completely disassembled the tumbler, sear, etc, but again found no broken or worn parts. Reassembled, and the hammer would release on pressing the sear. Reinstalled onto the rifle, and the trigger released the hammer. Horrendously heavy trigger-pull, but the trigger was frozen solid before.
FWIW, I was expecting the hammer to be further back at full cock than it is, but now it releases properly. In short, wouldn't work, so I took it apart and couldn't figure out what was wrong with it. Then I reassembled it and it works fine. I hate it when that happens. Would be happy to post pictures of the guts, if anyone is interested, but I have yet to figure out how to post pictures.
Anyway, again, thanks.
Griff