If I have this right, the late C.I. Bullets (and therefore the AVL Classic with the same tinware) have the air filter in the right-hand toolbox and the old air filter box is thus an empty resonator, with a second pipe to the toolbox where the air filter now lives. I would agree with the other comments that a previous owner has converted the resonator box back to a filter box and simply not bothered removing the now surplus item from the toolbox.
I quite like the look of that retro-fitted oval filter on the Electra-X, which would also work with the AVL Classic. The Electra-X's original side panels (steel, not plastic) and the sausage box tool holder were taken from the 350 AVL Thunderbird model. I have seen a few cases where Electra-X owners in the UK and Europe have fitted the old-style tin ware, as the AVL Classic was never marketed outside the USA.
A.