Considered as a cachinnans x argentatus, and probably a F1-hybrid.
Two flight shots, showing the upper- and underwing:
Probaly the most striking feature in this individual, saying that this can't be a pure Caspian, is the pale inner primaries. The four innermost primaries (P1-P4) are showing dark only on the tips, and pale outer webs, which creates the impession of a pale window on the inner hand. A pure Caspian Gull would typically show dark outer webs on the inner primaries, giving a so called "venetian blind pattern".
The underwing is pale, sparsely and neatly patterned, and could have been found on a pure Caspian as well.
On the right photo, note the shape of the body with the long neck, resembling Caspian.