[Andre]

i don't know the exact answer, but I suspect that since we can't get the gas that was used in the 60's, the slight difference in the carbs is lost to the ages

[JSuchma]

Vacuum port for the automatic transmission modulator.

[mgarblik]

I am sure someone like Cliff could provide a very detailed answer, but a few generic differences would be: 1. Auto trans carbs typically have a few additional vacuum ports in the base plate or main body for things like the vacuum modulator and such. 2. Auto trans carbs often have a hot idle compensator for cleaner idle in drive in hot weather driving. Not aware of manual trans carbs having this feature. 3. In general, an auto trans carburetor needs an idle circuit tuned to allow smooth operation under light load from being in a drive gear with torque converter load. I am sure there are other subtle differences.