Andrew Gabriel wrote:

Year of manufacture Expected lifetime
1958 42 years
1964 20 years
1970 10 years

It would seem the industry cleverly worked out how to
turn radiators into consumables throughout the 1960's,
and create the need for inhibitor (another consumable).

Active/acidic flux wasn't widely used, probably until sometime around
the '70s (?). Until then you had to use a passive flux and thoroughly
clean the pipe and fittings before soldering. A couple of old-timers
have told me that the raging corrosion, and the need for power flushing
to shift corrosion products obstructing the pipes, is something that
they rarely/never saw when they were youngsters. I suspect the active
flux is a major cause.