"your bird shows extremely limited black on p6 (restricted to outer web only), which I would consider outside of the variation in smithsonianus. I've certainly never seen this in colonies of argenteus (which generally shows a bit less black on primaries than American Herring). Could well be a hybrid or backcross (with Glaucous Gull).
Some additional shots taken on February 22nd, 2016 at Unity Park, Turner's Falls, Franklin Co., MA.
Here some images of the bird, all taken on February 17th, 2016 at Unity Park, Turner's Falls, Franklin Co., MA.
This last shot is the only other adult Nelson's Gull that I've seen relatively locally.
It was at the Windsor/Bloomfield Landfill, CT on March 4th, 2009.