I cleaned up my rear carb today and took a few pics.
First off, here's what's left of the original foil ID tag. Part number
7013065 tells us it's a rear carb used from 1959 - 1962 on both manual and auto trans 389 engines.
Top view of the carb where you can see the large vacuum diaphragm that was used to actuate both the front and rear secondary carbs depending upon engine demand (wide-open throttle).
That vacuum actuator is huge (camera perspective makes it look even bigger), it has to be in order to pull those two carbs wide-open.
A look at the linkage from the actuator to the throttle shaft.
I removed the actuator since I'll probably be using 1965 - 1966 manual-trans mechanical linkage instead. All Pontiac Tri-Powers had vacuum actuated secondary carbs up until the introduction of the 1965 GTO stick-shift engines. All GTO auto trans Tri-Powers were vacuum actuated.
A lot of Tri-Power owners didn't like the scary one or two seconds that the vacuum-controlled secondary carbs took to close after lifting off the throttle, and who could blame them. Many were converted to aftermarket mechanical linkage and some owners had the Tri-Power setups removed altogether and replaced with a standard single 2-barrel or 4-barrel.
I checked the angle of the throttle plates at wide-open position and found that they don't open all the way. Very likely this was engineered into the setup to keep from adding too much airflow for the engine to handle.
Here's the stop on the side of the throttle body.
