Gull spec, probably smithsonianus - Azores, March 2007. (Thys Valkenburg).
And comparison to Nelson's Gull (Bruce Mactavish).

(20 March) Unidentified gull on the Azores. Could this be a pale Herring Gull (smithsonianus) or maybe a Nelson's Gull (hyperboreus x smithsonianus)? All comments much appreciated.

thys_valkenburg 'att' yahoo.com

(22 March) Thanks for reactions on this unID-ed gull. Sorry for not putting more details on it's age in the first caption, but Martin Reid nicely explains:

Most likely this bird is a pale smithsonianus HGu. Bruce Mactavish, who sees and photographes Nelson's Gulls in Newfoundland commented:

I would vote for a pale, sun bleached 2nd winter (3cy) smithsonianus. The structure of the bird fits a good sized smithsonianus. I don’t think it is a hybrid Glaucous Gull because and of the bill pattern and extensive dark in the primaries. The dark of the tip area extends along the cutting edge of the bill toward the base. This a very common bill pattern for 2nd winter smithsonianus. Typical 1st and 2nd winter hybrid Glaucous Gull x Herring Gull (in Newfoundland at least) have a Glaucous Gull patterned bill with a sharply demarcated inner edge to the black tip area. The folded primaries are extensively dark with little or no sign of fading is typical for 2nd winter smithsonianus. Typical 1st and 2nd winter hybrid Glaucous Gull x Herring Gull in Newfoundland have the outer most four or five primaries a shade of brown about half way between a Glaucous and Herring Gull. Typically there is a relatively broad pale fringe to the tip outer five primaries easily visible when at rest. The Azores bird shows small pale tips, common for 2nd year smithsonianus.

Below, 4 images of Bruce have been added.

And Martin Reid continued:

While it might be a Nelson's Gull - the name given in North America to hybrids between American Herring Gull and Glaucous Gull - it is hard to rule out a very pale American Herring Gull. I see birds like this occasionally at this time of year in Texas, and I basically go on flight-feather coloration to make a call on whether it's a pale AMHE or has some Glauc in it. Your bird appears to have primaries of a color I'd expect for AMHE in March - however the secondaries (just visible below the greater secondary coverts in the first image) seem too pale for a pure AMHE. My best guess is a backcross AMHExNelson's Gull.

The short-winged appearance may resemble Great Black-backed Gull or some influences (GBBGu x AmHGu) from that taxon, but the long, slender bill and sloped forehead better fit Herring Gull. A pale local Azores YLGu can probably excluded by the lack of a uniform saddle; in this individual the scapulars are "2nd-gen like" patterned with a ponounced shaft-streak, lacking adult-like uniform feathers.

Everyone who commented, thanks a lot!