A couple of points:

1) My copy of ASTM spec D910 page 0539 states that the allowable TEL
content of 100LL avgas is 2 milliliters per gallon NOT 2 grams per
gallon. This means there will be over 1/3rd pound of lead in an 80
gallon tank of 100LL, plus about 1/5th pound of ethlylene dibromide
scavenging agent.

2) My muffler "failures" were from the internal flame tubes, which
must be present and in reasonable condition if they were originally
present when new, but are not required on later mufflers that did not
have the internal flame tubes. We did a proctoscope exam with a small
flashlight at annual to verify their condition. Our engine was run
about 50 deg rich of peak, or so we thought as we had only a single EGT
probe. There was little engine roughness though if the mixture was
leaned further. Manifold pressure was set to 21 inches for cruise,
less at higher altitudes.

I suspect many 172s were just run until the flame tubes were all gone
out the tailpipe.