chocolate-morph Ring-billed

Caleb put a photo of a Ring-billed Gull with all dark wings on the Facebook Gull page.  I happened into a pretty similar looking bird at Tiscornia this afternoon. 

Gulls are massing between the piers feeding on small fish; I assume steelhead heading out into the lake, but that's just a guess.  It's a great time of year to pick up Laughing or Franklins, and we have a few California Gull records for May (to say nothing of the Glaucous-winged) so this bird drew my eye.  Here's a montage of the individual at medium distance:

It circled around and flew past a couple times.

There's only a few gray feathers in the entire upperwing, and I'm not sure those aren't new feathers growing in.

A view of the underside:

It actually has a bit of a belly patch.

I know there's pics of Little Gulls like this in the books, but I'm not sure I've seen this variant illustrated for Ring-billed.