I didn't see this individual in flight nor did I see the underwing so unless it gets re-found and flight views/better images produced putting it in the box of either dark argentatus or smithsonianus with any certainty is difficult but from what I saw I believe it is at least as good a candidate as this though to provide some balance see here and anyone seriously interested in proving/disproving it may find this useful.
Compare the more typical 1st-winter 'Herring Gulls' left and below in the 1st image. Note the bill in the 2nd shot above, the basal half beginning to acquire a pinkish base.
1st-winter, dark almost uniform 'chocolate' brown, lacking much tonal contrast between upperparts and underparts. Underparts showed some paleness around lower throat/upper breast but otherwise fairly uniform through to vent. Mantle/Scapulars showing some 1st-winter feathers, dark-centred (black) and fairly plain with little in the way of pale edges. Juvenile coverts dark, again showing little by way of paleness or barring. Greater coverts plainish brown based on outers with paler buff edges; inner greater coverts showed some barring.
Tertials dark centred with fairly narrow pale fringes, towards feather tips, the tertial step was obvious.Primaries black with faint pale tips. Tail-band appeared solidly dark and uppertail coverts were heavily barred as were the undertail coverts. Bill parallel edged without strong gonys angle, developing pink-ish base towards basal half. Legs dark pink