There are a few loopholes in the law. You are correct in your reading, that is how some of these "non approved" refrigerants get by. They are not approved for retrofit from R-12, but are ok for retrofit from R-134a. So if a vehicle is changed from r-12, to 134a, first, it can then be changed to the non approved refrigerant.
There are many states that have laws against hydrocarbon refrigerants, in motor vehicle use.

My personal opinion, is, use only R-12 or R-134a in a MVAC. Main reason, is, component warenty. Warenty with any manufacturer I know of, is only good when used with R-12, or R-134a.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Yup, and those loopholes are exploited to the max by the marketeers of hydrocarbon refrigerants, along the lines of "Warning, caution, check your local laws (wink, wink) before using our stuff to replace R12! (nudge, nudge, wink) Our stuff may be used to replace R134a freely (wink, hint hint), so if you have any doubts about the legality (ahem, cough) of our stuff, you will want to convert your vehicle to R134a and then put our stuff in (wink)." It's a crock. And just because there's no Federal law at the moment against using explosive refrigerants in place of R134a, doesn't mean it's OK or a good idea. Just means it's not Federally illegal. As Charrlie_S mentions, many states have laws against hydrocarbon refrigerants in motor vehicles...for a very good reason.
That's one good reason. Another: When your A/C breaks and you're away from home, or you don't have time or space or knowledge to do the repair yourself, a system charged with anything but 12 or 134a is going to be considered "contaminated" by most any shop. If you're lucky, they'd agree to work on your car and charge you a very hefty decontamination fee. It's an expensive damnuisance for them when they have to use their equipment on a contaminated system, so it's reasonable and proper for them to expect the customer with such a car to foot the cleanup bill.

More likely, though, they will simply refuse your business.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

A/C is still working fine.

This is the last time I'll post on this thread, but the a/c is still going strong. And Dan, if this makes me selfish get a load of this- I use two ply toilet tissue. I'm so ashamed of myself. I should'nt have written that. Oh well.