The bird is obviously in wing molt. Two different experienced observers in Europe have said privately to me that the bird is exactly right for L.a. argentatus, down to even the state of molt. The large white tip to P10 is a mark for L.a. argentatus.
The bird has a quite rounded head for smithsonianus, and a rather slight bill. Some smithsonianus, presumably females, look like this. In my experience, I would guess maybe one smithsonianus in five has a head and bill like this. But such birds are invariably (in my experience) noticeably small (in overall size). This bird was not. It was hanging around with 10-12 Herrings and was definitely not a small bird. That's one of the things that I noticed right away about it. I didn't carefully assess the sizes of the birds, but I saw no obvious size differences among any of them.
There is an extensive discussion of this bird on the responses page.
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