Petcock is P/N 16957-333-025. eBay lists some unbranded unit
here and 1 genuine NOS Honda unit (in Australia)
here.
The parts diagrams for '72 & '74 CB350F both show that stock slow jets were #35, and #38 was optional. The only one available
on Partzilla is the #38. JetsRUs doesn't even have a listing for a #33. Even Randakk's doesn't have a listing for a slow jet that small. I don't know enough about the CB350F, but I'm wondering if there even are slow jets that small.
Why do you need that specific size?