Considered as a cachinnans x argentatus, and probably a F1-hybrid.
A flight shot, showing the upperwing and the spread tail:
1.
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".
2.
Tail pattern like a pure Caspian Gull, with a rather broad, blackish and well defined terminal band, tapering on to the sides. Rather sparsely patterned upper tail coverts.
English version