Odd Duck - Help identify!

This afternoon as I left work I saw a group of what I took to be young mallards (the juveniles all look like females until the males acquire their adult plumage) on the ground near the river's edge. These are very pretty in a subdued sort of way - note the soft buffy coloring, the bluish-purple splash of the speculum, and the dark streak that passes through the eye and tapers toward the back of the head. But I soon noticed that one stood out as looking quite different.

The duck centered, above, was considerably darker and less obviously variegated than the others, had darker legs, and bore a blotchy white section on the front of its neck as well as some white at the top of its bill.

After some consultation with my resident bird expert, a field guide, and some internet searching, our best guess is that this might be a juvenile male northern pintail in the process of acquiring his adult plumage. Adult mail northern pintails have a striking brown head and white neck, and are also known for the long, elegant tail feathers that give it its name. Or could it be a mallard x pintail cross? Why else would it be in the middle of a flock of mallards? Or is it something else entirely? I welcome your thoughts/guesses/educated opinions!