Mufflers are one of the most failure-prone parts of internal combustion engines. Just be glad you are not living in the days of leaded fuel. The Tetra Ethyl Lead (TEL) is transformed into metallic lead during the combustion process, and lead being a conductor, this was a problem when it deposited itself on spark plug insulators. To overcome the problem, TEL was always packaged with "lead scavengers", extraordinarily corrosive chemicals, mainly halides and bromides. The halides and bromides ate exhaust systems. In those days, cars used to require regular replacement of mufflers, and even exhaust pipes.
Thank you for the offer to keep us informed as you investigate the condition of your three engines, and hopefully turn them into two good ones. The usefulness of these threads to other members depends on them incorporating clear, relevant photos like the ones you have posted above.
