The part you are looking at is the Aneroid/Mixture Control (M/C) Solenoid or the Primary Enrichment assembly. That black plastic cap is a protective cover for the Lean Mixture Screw, which fine-tunes the travel of the primary metering rods.
The "Double D" Dilemma
You aren't imagining things—it is indeed a Double D head. GM started using these to prevent DIYers from messing with emissions settings.
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The Size: For a 1979-era Rochester, the standard size is 2.5 mm (or .090") Double D.
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The Tool: You do not need a $140 kit. You can find individual "Double D Carburetor Adjustment Tools" online for $10–$15.
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The DIY Hack: Many people use a small piece of 3/16" copper or brass tubing. Use pliers to slightly flatten two sides of the tube until it fits snugly over the Double D head. Alternatively, some find that a very small, thin-walled deep socket or even a wire connector (crimped) can grab it.
The Procedure: What You’re Missing
The black plastic cap is often pressed on or held by a small tab. However, the screw itself is what holds the tension on the metering rod hanger.
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Don't Force the Cap: The cap usually pops off with a thin flathead screwdriver once the tension is off, but sometimes it's easier to unscrew the bolt through the cap if it's open-topped.
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Count Your Turns: This is critical. Before you remove it, turn the screw clockwise until it lightly seats and count exactly how many half-turns it took. This allows you to return the truck to its current "tune" when you reassemble it.
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Removal: Once you've recorded the position, unscrew it completely. The assembly (hanger and rods) is spring-loaded, so keep a finger on it so the parts don't launch across the garage.
Is it just a bolt?
No. It is a metering adjustment screw.
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If you just pull it out and put it back in at a random depth, the truck will likely run "lean-stumble" or "pig-rich."
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It controls how deep the tapered metering rods sit in the primary jets at idle/part-throttle.
Pro-Tip: Since this is a '77 Farm Truck with a '79 carb, check if there is a metal plug blocking access to the base of the carb (the idle mixture screws). If those are still plugged, someone likely never did a full rebuild on this unit.
