I didnt think it was designed to be anything but an AA gun from the start? Obviously, it turned in to something far greater than "just an AA gun".
I think having it stationary at a field would lend more towards it being AA than dual purpose, but that is just me. Did all the units who deployed this weapon receive AP as well?
Early as 1940 it turned into an Anti Tank gun - being so early in the war, shouldn't it of been duel purpose or do we exclude it because it was "originally" designed as an AA gun? If so then we do we have modified aircraft that served throughout the war? Look at the P47 line, clearly a fighter from the start, but turned into a ground attack plane.
I would argue if the 88cm was turned Anti-tank late in the war, but being so early in the war, its almost a given it will be duel purpose, perhaps like a 5 inch gun, able to fire one way or the other.
Here's a posting of the earliest use of the 88cm as an Anti-tank weapon:
"or the 1940 Battle of France, the army was supported by eighty-eights deployed in twenty-four mixed flak battalions.[4] The eighty-eight was used against heavily armored tanks such as the Char B1 bis and Matilda II, whose frontal armour could not be penetrated by the light 3.7 cm anti-tank guns then available. The 88 was powerful enough to penetrate over 84 mm of armour at a range of 2 km,[7] making it an unparalleled anti-tank weapon during the early war, and still formidable against all but the heaviest tanks at the end of the war. Notably, Erwin Rommel's timely use of the gun to blunt the British counterattack at Arras ended any hope of a breakout from the blitzkrieg encirclement of May 1940. In the entire Battle of France, flak destroyed 152 tanks and 151 bunkers.[4]"
I can account one of rommel's books he used it exclusively as a bait weapon, sending tanks in then having them disengage and retreat - while the 8.8's were used in ambush.