The late 1950's early 1960's GM factory tripowers had the edges of the throttle plates on the end carburetors only coated with a product called DAG-213.
This is NASTY stuff, and probably should not be used by the general hobbyist (we WON'T use it, and have been rebuilding tripowers quite successfully for more than 40 years!). The DAG-213 was used as a time saver; as the individual centering the throttle plates in the throttle body could be less precise (time saving). This stuff really isn't necessary.
One other issue those who would attempt to use non-tripower carbs for ends should at least be aware of. A normal (not tripower end) carburetor is never asked to completely seal; there is always some clearance (maybe .015~.020 inch) at idle. Thus thin throttle plates are used. The factory tripower end carbs, which must completely seal, used MUCH thicker throttle plates to prevent binding in the throttle bore due to heat expansion when they are closed cold.
As to the holes in the throttle plates, these are used for two totally different issues in different applications.
The most common applications are holes from about 0.060 to 0.090 used on primary carbs only; generally either large engines, or engines utilizing cams with lots of overlap. The purpose is an inexpensive idle air bleed system. The holes allow a certain volume of air to bypass the normal path around the edge of the throttle plate. This allows the throttle plate to be closed sufficiently that the air velocity past the throttle plate is increased; causing a better signal at the idle and off-idle discharge ports.
Some factory GM tripower end carbs (generally those with auto transmissions) had much smaller holes (about 0.020 from memory). These acted as a bleed to bleed off pressure in the throttle bores of the end carbs due to heat. This pressure would be in the form of fuel vapor, and would create an instantaneous "rich" hesitation when the end carbs were opened. The holes minimize or eliminate this condition.
Jon.
