I called the Denver Air Resources Board about that, an engineer explained it's only the sale of 49-state cats that has been made illegal. If you show up to the test station with a federal converter they won't fail you because of it. The guy literally said "You could drive up to Wyoming and buy a 49-state cat, install it in your car, take it in for testing and there would be no issues."
It was enacted mainly to protect drivers who don't know any better and would buy cheap cats that would only last 3-4 years, or get screwed by a shop overcharging them for a cheap generic unit. I'll keep putting cheap federal cats in my cars that need testing lol. I think you might even still be able to order them online depending on the vendor.
And the emissions testing regulations started with 1976 model year because that's the first year cat converters were mandated for all cars in the U.S. Some cars may have gotten them a year or 2 before that. I did once long ago have a slant-6 Duster parts car with a cat but I can't remember if it was a 75 or 76 model year.
EDIT: CO has also recently relaxed emissions testing on 1976-1985ish vehicles, buddy of mine has a 1983 Chevy K20 truck with no catalytic converters AT ALL and he didn't get failed because of it. I've been helping him tune it to pass the sniffer test as he's not familiar with older carb'd engines. Also the CO front range has a bad ground-level ozone problem in summertime that's exacerbated by vehicle emissions.