The muffler and tailpipe were dipped at the factory in a liquid ceramic material and then fired at a higher temperature.
After that, the muffler was wrapped in asbestos followed by a galvanized steel shield.
So sure was AMC that the muffler and tailpipe were ironclad, they were guaranteed against rusting through as long as the original owner had the car.
This was done as early as the 61-62 Rambler, and as late as the 66 marlin
https://www.hemmings.com/magazine/hcc/2005/08/1961-1962-American-Motors-Rambler-Classic-Custom-Six/1281424.html
http://hooniverse.com/2017/07/28/the-carchive-1966-american-motors-marlin/#more-243577