Today at Fonte da Telha, Portugal, when looking for ringed LBBG, I have spotted what it seems a 4cy Larus argenteus, in the middle of LBBG flock, and also some YLLG.

Although it is not seen in the photos, it had pale white yellow iris, very pale yellow base to the bill with light orange gonydal spot at the lower mandibule, contrasting with the black marks on both mandibules as seen on the photos.

The legs seemed, flesh colored, and with at least 3 new and short primary feathers moulted: 3 large mirrors although only 2 of them are seen above.
When flying, the bulkier and chubby look was also seen, giving the impression of larger bodied gull with short wings....at least when compared with YLLG.
I would like to hear your far more experienced opinions!!!
all the best,
Rui