I had a similar bird at Mai Po in April 2015, which was photographed by Cherry Wong. It was not quite as advanced into breeding plumage as this bird, and was not as close as your bird seems to have been. I think there's a very good chance this is actually the same bird returning to Mai Po as a stopover again.
At the time I thought that bird was probably Cox's Sandpiper, and from discussions I had with others that also seemed to be the most likely identification anyone suggested. I realise now that I did not actually submit the record to the RC, which perhaps I should have done.
There is some discussion about that sighting, and photographs of the bird, on BirdForum at
http://www.birdforum.net/showthr ... highlight=sandpiper
Congratulations on getting these photos of a very interesting bird!
