I don't have the definitive answers you seek.. But, I'll share what info I have.
Below is the booklet cover and pages used by Honda that I have. They were posted to this forum way back by (I don't know who). I don't have the CB500 pages.
Regarding emissions:
The EPA made the first actual mandates for the 1978 model year. (Chart below from the EPA site)
I think it reasonable to assume that Honda was prior informed by the EPA, knew these mandates were coming, and began making changes to their machines, to see how close they could come to the specs by doing smaller changes to their machines as the years got closer.
I see the first change in 1974 US model year, with the introduction of the engine crankcase breather on the CB550.
Of interest, is the molded in numbers on the newly introduced Air cleaner box which is CB550-F.
The actual F model wasn't introduced to the US market until 1975. But, not only did that model have the crankcase breather system and the same air filter box, but it also had the 4 into 1 muffler system that required the carbs to have leaner jetting and solid tipped air bleed screws for a more direct response to adjustment position. The F models have slightly better MPG characteristics which is probably an artifact of the lower hydrocarbon output levels. However, these changes could not make the F model configuration meet the 1978 mandates, so the model was discontinued for that year. The CB550 F model only existed for US years 75-77.
Interestingly, the 1976 CB550K model had a change to the engine breather as a side step around the EPA exhaust mandates (still not enforced, but under their watchful eye for "spirit of cooperation".) To reduce the hydrocarbons coming out the exhaust, the hydrocarbons were dumped into atmosphere directly, rather than out the exhaust where the sniffer monitored for total hydrocarbons. The old style carbs also have a different set up (087a). I have yet to see a physical example that was any different internally to the 022A set up. But, then I don't have access to an unmolested 087a set, either.
In 77, the K model was revamped with different carbs (PD type with an IMS instead of the air bleed pilot screw) and a different 4 into 4 system with higher back pressure/smaller outlet size exhaust. This allowed the lean burn mixtures at the carb to produce exhaust emissions within the EPA mandates even with the Crankcase breather system feeding hydrocarbons back into the engine induction. However, the 78 CB550K was the last of the CB550 line in the US.
Of interest, is the 79 CB650, squarely under the scrutiny of the EPA. Quiet exhaust (higher pressure) and PD style Carbs, but like the 77-78 Cb750 models, accelerator pumps were added. This allowed the idle mixture to be leaned to the max, yet still get the required throttle twist response.
To bore you further, mechanical slide carbs were gradually phased out in favor of CV carbs, presumably these are easier to bring within EPA mandates, without accelerator pumps shooting raw gas into the carb throats for throttle response and enraging exhaust sniffers.
