... So it *was* a bad breaker. BUT WHAT CAUSED IT TO BURN OUT IN THE FIRST PLACE??? ...
Back of old breaker. Contacts are in crumbles.

Have a feeling this is the reason. Why? Because it makes sense.
Answer to another member with same problem on an old DIY forum:
"I would suspect a loose connection between the breaker and the bus bar. This can be caused by a breaker that is not fully engaged. A loose breaker can be made worse by a humid condition or other corrosive atmosphere.
"The loose connection allows a thin layer of oxidation to build up between the contacts which increases the resistance of the connection. This resistance causes heat build-up which increases the rate of oxidation until the heat becomes so much it melts stuff. If the connection is so loose that it causes arcing then the heat builds up very quickly."
May have lucked out again. If I can buy another 22 years with new breaker, that should be good enough.
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