I know. It seems to be very hard to find clear photos of that location.
It was this photo of Formidable that prompted me to look closely at this location on Illustrious. The silhouette of the (shadowed) rearside of a set of 20mm shields there seems clear, and to the immediate left of them the silhouette of some box mounted on the railings at the end of that external walkway.
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z Formidable 10th 20mm - Copy.jpg [ 40.55 KiB | Viewed 10354 times ]
In your photo a similarly positioned box, the railings (particularly the third one down) and the chap standing there in a duffelcoat obscure and distort things but I believe that you can nevertheless discern the top and left edges of the left shield, and (in the dark) the right edge of the right shield.
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z Illustrious cap015 - Copy.jpg [ 103.79 KiB | Viewed 10354 times ]
In the 1941 USA repairs a new protected area/half tub appears there where none had been before. In the US dockyard starboard broadside photo taken 24 11 41 the back of it can be seen in silhouette with something in it of the right dimensions - but whatever it is was under a tarpaulin/canvas cover (as are other 20mm in that photo).
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z Illustrious 1941 11 24.jpg [ 12.2 KiB | Viewed 10354 times ]
A detail from the extreme right edge of a photo taken during the Firebrand trials seems to show the right shield fairly clearly. It is a pity the camera was not aiming slightly more to the right!
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z Illustrious 1943 2 8 Firebrand trial Clyde.jpg [ 19.4 KiB | Viewed 10354 times ]
In one of the Feb 1943 de-ammunitioning photos taken from the flight deck the dark horizontal of the 20mm barrel is fairly clear. We are side on to the shields again but I believe that you can just about make out a light sheen of their angle.
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z Illustrious 1943 2 22 de ammunitioning.jpg [ 7.44 KiB | Viewed 10354 times ]
(It is perhaps also relevant that this location was clearly thought suitable for light AA as it later gained a sponson on which were mounted two twin powered Oerlikons.)