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This is the correct wording for the HL4 option.

HL4 24 gals.---In Cab behind seat---For use with 131" wb. Conv and 165 wb. Crew Cab Sweptline pickups only. Not available with Hinged-Back Bench Seat.

The standard tank was frame mounted in front of the rear axle. The HL4 in cab was in addition to the factory frame mounted tank.

I ordered my 77 reg cab short box 4x4 new and I could not get an auxiliary tank. I added the new tank before I drove it. The bench seat with the fold down back would work but the release levers had to be bent on a 45 degree so they would not rub through the tank. 30 seconds with a 12 inch crescent wrench fixed that.

Again if someone can prove they had a factory short bed regular cab put up the documentation. Just because it has one now does not mean it rolled out of the factory with one 36 years ago.

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They are out there. Biggest way to tell if factory or not is the nice Metal decal next to the switcher valve switch in the dash below the ac controls... if it is there and says "STD in AUX out", then it is factory. There was no replacement part for that decal... if its got that decal, and no in bed spare tire mount, then its factory.
But what do I know...I'm friggin stupid when it comes to these trucks....