What age is this Common Tern?

While surveying Pointe Mouillee State Game Area on Saturday we came across a strange medium-sized Sterna tern which I am still trying to age. I am sure the bird is a Common Tern, but I am not sure how old it is. At a glance it looks much like a basic adult Common Tern, except that the lower breast and belly are gray and the cap is fully black. So I'd just call it an adult except that it has an ALL-BLACK bill, carpal bar, and dark outer primaries much like those of a 1st summer bird still retaining its juvenal outer primaries (though not as dark/worn as most 1st summers). So we appear to have a montage of features not normally shared by any one age class. To me, the bird is likely an advanced 1st summer bird which has developed an all-black crown (most if not all have less than a fully black crown according to Maling and Olsen [Terns of Europe and North America]) and gray underparts. Or could it be a retarded third calendar year?

The possibility of a longipennis Common Tern (the subspecies in the eastern Old World) was considered, a contention seemingly supported by the all-dark bill and dark legs, however this subspecies in adult plumage would not have a dark carpal or dark outer primaries, so it seems unnecessary to invoke this. Thoughts would be appreciated.