I see that Honda parts only go back to 1977.
Some dealers have complete Honda fiches -- I heard from someone else they could only find until 1977 too, but I don't know why that is or where you are looking. If you go to Western Honda (
www.westernhonda.com), which is my local Honda shop in AZ, and click on OEM parts then OEM parts fiche, you can enter any year up to 1959 for parts fiches, part numbers, and lists of parts that are still available. They've got the 1972 350 carbs, I looked, and was surprised to find the carb pistons available (again? or have they always been...?). You won't find your butterfly, but if you need anything else (oem jets, e.g.) you'd be surprised what you might find still available.