Depends entirely on WHICH Rochester B. As Tom (above) mentioned, some used 12x28. Others used 10x32. To know for certain, either measure with micrometer, or look up bill-of-materials based on tag number.
The fillister head is the thicker head (withstands more torque).
Finish on 1950~1955 was yellow zinc. Finish after 1955 was black oxide.
We quit selling the screws as we no longer can purchase "made in USA"; and the off-shore versions seem to be inferior quality.
One of the other issues with the Rochester B's is the zinc alloy was softer than that in other makes of carburetor. Have seen lots of B's that required either drilling and retapping to the next larger size, or installation of heli-coils.
As Bart suggested, a thread pitch gauge is a useful tool; and if you have to have off-shore, the off-shore dial calipers are quite inexpensive. Personally, I prefer Starrett.
Enjoy.
Jon.
