Hi lou I made my inicial ssp. identification based on P. adriaens & B. Mactavish paper
Identification of adult American Herring Gull (I Know that should not be used the other way around), figure 6 and pattern c on P9 are present on 67.9% of teus versus 5.7% tatus. I am assuming that P9 on the photo as a small withe mirror in the inner web with white pale tip, but lattely my primary feather evaluation is not at is best, lou

. But like you said, mantle color and still growing P10, are better for northern.
Rui