Went to replace the mainspring, and this is what was inside...

Yikes!

I'm guessing that made some noise when it went. Pretty interesting fracture pattern!

I dunno - my 1st question was:

Where is the big chunk of the missing spring?

Like someone went to replace it, took the chunk out, and closed it back up - LoL!

Thought you were a forensics guy--This is CSI for a metallurgist.

I'm betting there's no metal missing, the mainspring just cracked/exploded along a straight line. My guess would be that someone scratched/gouged the spring with tweezers or something that concentrated the stress along that line so it almost all snapped at once along that "fault" line. Another possibility, but can't quite figure out how, but there would be a brittleness along that radial line.

You could verify if there's material missing by matching up the broken ends. If they don't fit together like puzzle pieces, there is material missing.

Interesting failure analysis. Thanks for posting.

We have had discussion about this in the past. One post is;
https://ihc185.infopop.cc/eve/f...933998377#6933998377

Unbelievable or unbreakable?