A pale 1cy Slaty-backed?

*Edited: Thanks to Mr.Ujihara of the Japanese Gull Site for identifying this juvenile bird. He said that it's either a Vega or a Mongolian Gull but not a Slaty-backed Gull because of its very pale mantle. He made a comment that it might be a Vega because of the brownish coloration on the coverts.

This strange juvenile bird showed up at the beach last Saturday, October 17. It has pale whitish plumage similar to those of the 1cy Mongolian Gulls, but what make it look more like a Slaty-backed are the short primary projection look and stout, bulky shape. I don't know if it's normal for Slaty-backed Gulls to have this kind of pale juveniles or not, since there are not so many Slaty-backed around here in the south and I've never seen a picture of a juvenile Slaty-backed Gull in this form before too. The markings on its mantle is so pale that it almost looks like the new plain greyish feathers have already grown.

The primary projection and tail are very short comparing to the Taimyr Gull in the front.

The primaries also look blackish comparing to most juvenile Slaty-backed

The mantle is very pale and plain whitish-grey looking.

Comparing to another 1cy Slaty-backed Gull on the same day

The shape is pretty much typical Slaty-backed Gull (comparing with a Vega on the left)

Some more preening shots

The feathers seem to be very fresh.

Tail band doesn't seem to be as narrow as typical Mongolian.

The same typical 1cy Slaty-backed Gull as above

It looks a bit different in warm evening light.

Then the gulls were chased away by a gang of Large-billed and Carrion Crows,
so I could get some shots showing its upperwing details.