Catalytic Converter

Like to throw this out for opinion as the Internet is as contradictory as my wife.

Can anyone definitively tell me the first year catalytic converters became mandatory. I have researched they first appeared in 75, but were not mandatory until 76. Then I see anything 75 and on they are mandatory.

Little help. I am looking to upgrade my 75 exhaust and if I can dump the converter I'll be a happy boy.

bv