The numbers you are refering to are set up numbers (657A, 657B). I don't know specificaly what the internal difference is. But, using the Cb550 carbs set up number convention as a Honda modus operandi, there can be differences in the:
main jet size,
slow jet size,
slide needle profile (taper and diameter),
main emuslion tube cross drilled hole size and placement,
pilot screw taper and design features,
and main air jet and slow air jet (drilled into the carb bodies).
If the rest of your bike is stock, then the stock set up (657B) should be correct for your bike. And, all four carbs in the set should have the same setup.
It seems reasonable that if you transfer all your internal parts from the 657B carb to the 657A carb, it should then be set up the same as the 657B and operate the same way. Were it me, I would compare the air jet sizes with the appropriate drill bit shank.
A bit of confusion...
The Honda shop manual says you should have 7A carbs for a K4. However, it doesn't mention the setting mark for K5&6. I'm not familiar enough with the CB750 to know what the differences are between K4 and K5. Is it the exhaust system? If so, that might explain the set up difference. But, then you say you have a 74 model which should be a K4 and the shop manual says 7A is the set up for that. Are you certain your bike is a 74 model?
I would assume there is some subtle difference between 657A and 657B. But, I don't know what that is.
A bit of searching and I found this...According to :
http://hondachopper.com/carb.specs.htmlthe only difference between the two carb numbers is the main jet size. Could be. But, I think I would still check the air jet sizes, too.
Good luck!
