Like a number of gulls which are rare but occur with reasonable regularity further north in Japan, this would be a mega this far south. In fact there's never been an accepted record in Kansai as far as I'm aware.
The possible confusion taxa for any potential juvenile smithsonianus, are the super-variable Slaty-backed, Vega of course which is much closer in appearance to AHG at this age than it is to argentatus and also Taimyr Gull which may not be the first thing that springs to mind as a potential cause of confusion but it can look surprisingly similar at first glance.
As I said AHG would be a mega here but this gull seems to tick all the boxes, certainly a suite of features look good and there are three important points, the black outer primaries lacking pale inner webs, the dark greater coverts bar and the pale window in the inner primaries, which are wrong for Slaty-backed, Vega and Taimyr respectively.
Slaty-backed is the only one of the trio needing to be eliminated that often presents as structurally different. The SBG on the left has the typical deep-bodied appearance with hanging keel and stout, widely spaced legs. The bird in question is slimmer with a different head shape. The legs are also thinner, less widely spaced and less bubblegum pink. The dark coverts bar is quite prominent at this angle, something Vega doesn't show.
P8 is exposed here and SBG would show an obvious contrast between dark outer and broadly pale inner webs. Again the dark coverts bar is prominent and might suggest Taimyr but the just visible inner primaries hint at the large pale window on this bird, this would be darker and lack contrast in Taimyr.
Because I was digiscoping I was unable to get any flight shots and this is one of only two I have to show the spread wing. The window is nevertheless very obvious and contrasts strongly with the almost solidly black outer primaries on the upperwing. The underwing looks very whitish here and could be mistaken for SBG but the latter lacks the heavy dark framing and particularly leading edge. Any juvenile gull can show a whitish underwing in a single image like this. Below are comparative shots of two still fresh juvenile SBG taken in early October this year, one a pale individual the other darker. Though one is much lighter than the other the pattern of both wing surfaces is essentially identical; prominent venetian blinds throughout the outer primaries above and largely whitish below with a narrow, faint trailing edge.
The only other "flight" shot I have, which though blurry, again shows an obvious window in the upperwing and dark outer primaries below. The general paleness of the underwing continues across the secondaries which are obviously black on the upperwing. Incidentally, the adult behind it is Taimyr (evidenced more by the retained outer primaries than the darker saddle), to its right is Vega and further right still is Slaty-backed.
At first glance this juvenile Taimyr might recall AHG. It has a heavily saturated shawl, the flanks hint at the possibility of brown uniformity below and the vent / undertail coverts are heavily marked. But once airborne (below) the dark inner primaries give short shrift to that idea (7 Dec 2014).
Again alongside SBG, the head shape looks "Herring group" and the whole appearance is more elegant than the oafish SBG with its short, broad primary projection, drooping skirts and very deep body. The presumed AHG has far less white in the tertial tips than Taimyr, let alone Vega.
I don't have any shots showing the rump, uppertail and tail. In fact the former two were heavily barred and the latter mainly black but that's meaningless as Vega can be equally heavily marked. Again the overall structure is "Herring group"; the head shape looks subtly different to Taimyr with this gull having a long, straight forehead and flattish crown whereas Taimyr tends to have a steeper forehead and more rounded crown.
The vent is heavily barred throughout and the longest undertail coverts almost completely dark but again Vega can match this pattern. However it will not, in my experience, match this greater coverts bar plus I've never seen a Vega with so little white in the tertials.
A Vega in the foreground with slightly greyer, less chocolate underparts and showing far more white in the tertials. This particular bird is already acquiring its next generation mantle and scapular feathers so looks much paler above. Vega doesn't usually look quite as saturated with brown below.
Here with a full juvenile. But juvenile what? At the time I was sure this was Mongolian (and still believe it is) but I didn't confirm it at the time as I was too interested in the other gull. Juvenile Mongolian is very uncommon here, Mongolian is scarce altogether but most juveniles have moulted their mantle and scapulars before they arrive. I wish I'd given this gull a bit more attention.