I believe the reason why is because the automatics kicked the 2 outside carbs in with manifold vacumn whereas the manuals kicked in the 2 carbs with mechanical linkage arms.But I believe also like you said that the mechanical linkage didnt start coming on manual shift cars until around 1965-66 and werent on 59 cars with manuals so I would say they are only going on later cars with mechanical progressive linkage and dont really know the early cars were all vacumn operated outside carbs.Just my 2 cents on this.
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1969 Matador Red GTO Judge,1999 Ram Air Trans Am,1998 Daytona Pace Car Grand Prix,1959 Bonneville,2004 GXP Bonneville.
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