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It may have been a 65-66 thing. Posting numerous examples of build sheets of cars with A/C, but no HD battery. Have examples of window stickers with A/C and also HD battery. I also have the order punch card for a '66. It was ordered with A/C and HD battery, but there is no call out for anything other than the base alternator/regulator. Just going by the documentation, but it does not appear A/C, nor power top automatically triggered an HD battery (at least for 65-66).
Perhaps Mike can chime in since his un-restored honey gold convertible has AC, so he should have the original alternator with it. I dont know if he has the original battery that came with it.
Most of these documents other than 1 in Iowa are in warmer areas of the country. Just a guess but perhaps many cars shipped to parts of the country with freezing temps in the winter were ordered with HD batteries for more cold cranking amps.
His car has an alternator with a 50 marking on it. The window sticker calls out A/C and then also HD battery. Don't think he has the order info.
Both of my unrestored June 65 cars have purple stamped alternators and have heavy duty battery on the window stickers. I have the order sheet for the convertible and it also lists the heavy duty battery. My Aug 64 car has a purple stamped alternator. I don't have the window sticker for it ,but do have the order sheet. It lists heavy duty battery.
I think that is one of the more common color of stamps I see on 65 and 66 's on the show field.
Does power steering also factory into this as well?