A smog tech told me that owners are supposed to retain all emission equipment that are installed on pollution controlled vehicles, even if they are exempt from testing.
In short, your 1975 car is required by law to have a converter but they don't require inspections to validate compliance.
Correct, ALL cars in California regardless of year must retain all factory emission equipment. If your classic is spotted blowing out smoke and you get sited you will have to pass an emission test that includes the under hood check.
Whereas this is true I have never heard of it happening.