Thank you everyone. I have reached out to Jim Chambers via the email on his website, and have promised that in addition to paying for the part, I'll take better care of his components going forward.
Since somebody asked how it happened... I had drilled and tapped the holes for the lock plate screws with the lock completely disassembled. After inletting the side plate, I put everything together, with the lock disassembled. Easy. Then, for some reason, I wanted to see what everything would look like put together.... so I reassembled the lock, put it in place, and began screwing the lock plate down. What I didn't realize in my haste was that the screw, still not cut down to size, was protruding through the lock plate and pushing on the hammer. Ofourse it wasn't an issue when the lock was disassembled. I thought the screw was hung up and I gave it a hard twist, and pop... the hammer and the square part of the tumbler that engages with the hammer sheared right off before I realized what was wrong. So, new tumbler.
Embarrassing to type all that out. But worse things have happened, so we pay for a replacement part and we move along.
I have learned a ton in the last few weeks... prior to this project, I had assembled a CVA Hawken back in high school, about 27 years ago, but nothing like a real build. Until starting this project, I don't think I'd ever drilled a hole in a piece of metal, let alone counter-sunk, tapped for threads, etc.
Thanks. This site is fantastic.