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Battery Acid ? A reasonable suspicion ! Battery acid would certainly eat aluminum but as there was no damage to any of the carb-body venturi's or throttle-plates, this is difficult to imagine. Playing the devils advocate however: The battery is directly behind the carbs. The top of the battery is about level with the carb-throats and relatively close. So perhaps a "stretch" would be: No gasket between the upper air-box joint or "Pods" (or no filter at all) and a battery cap(s) missing. But how the acid vapour did not so much as "stain" the carb inlets, venturi, main-needles and jets .......? a mystery. The above "Battery-Scenario" seems really far-fetched but I've always believed." If you Think it can Happen - It' likely Can and Will". I did consider that perhaps a P.O. had stood the carbs up on end in some solution but that would mean the outside of the insert would have also been corroded.
Somewhat akin to the "Oak Island Treasure Mystery" it's difficult to speculate which one will be solved first.

I think it's a space junk scotch picked up on the way down skydiving. Just look at those little craters!

If you wanted to see "craters" you should have seen the outside of the bodies, lids and bowls. All three carbs looked liked they had spent a day on the end of a chain being dragged behind a logging truck !! No question that someone had thrown a hissy-fit trying to get parts "to part" and litterally beat the shit out of these poor things with the handle of a wrench. And not LITTLE nicks and dings, either. Chipped and missing pieces around the idle-needle bosses and the "fine" lip on the bodies on the bowl side.
Truly an example of the Dark Side of Tthe Moon !

What could possibly be accomplished by beating them? I do get frustrated some times but that's excessive.

I've seen carbs with small dings indicating the part was "tapped" lightly to release a gasket. I can not imagine in my wildest of scenarios how anyone could simply go "ape-shit" on these. And judging by the similarity of the marks - it was one owner. Hopefully that P.O. took up Lawn-Bowling right after this travisty.
Now - This is DEFACING !:

This is a Modification !:

I don't even know what to say about that but you are right... looks like they were dragged behind a truck. Could the bike have been in a crash and had the carbs ripped off and tumble down the road. Then sold as parts and put on another bike?