Rochester used a cold dip chromate solution to semi-protect the zinc alloy castings (airhorn and bowl). This solution is a mixture of nitric, sulphuric, and chromic acids; AND SHOULD NOT BE TRIED AT HOME! Chromic acid is NOT a nice chemical. This process was done, not to make the carburetor pretty, but to semi-protect it from oxidation. This process is more critical today with the alcohol in the fuel. The aftermarket paint offers no protection to the inside of the carburetor.
Rochester "parkerized" the cast iron throttle bodies. This is a "blacking" process similar to gun bluing.
Rochester electroplated steel pieces with "yellow" zinc. For those who are unaware, the steel would be electroplated with white zinc, and then dipped in a "yellow conversion coat". This is a finish that evokes argument among enthusiasts, as with time, the yellow conversion will disappear, leaving a white zinc appearance.
Rochester used the yellow zinc finish on steel screws up until about 1955~1956. Somewhere in that period, the standard for steel screws changed from yellow zinc to black oxide. This probably creates the most argument among enthusiasts, as the standard changed, but THE PART NUMBERS DID NOT CHANGE! I have many new old stock Rochester screws in the original see through plastic bags where two different bags with the same part number will have different finishes on the screws.
The yellow zinc electroplate disappeared in the early 1970's when a better finish (di-chromate) became prevalent.
Remember however, that the entire purpose of Rochester finishes was protection, not appearance. This differs with some other carburetors (ie the Chevrolet Corvair turbo) in which the finish was both protection AND appearance. The Turbo Corvair carb was painted gloss black after the chromate solution was applied, and all external steel components were chrome plated.
Jon.
