Oh, I like the idea of Keith (and, yeah, perhaps Broster) being face-blind! Also in the bits where he admires Ewen at amusing length, the focus tends to be on Ewen's body and manner rather than his handsome face (apart from the blue eyes, which I suppose are distinctive enough to stand out even without much face-recognition ability), which fits with that too.

I'm only slightly bad at faces, and the combination of identical uniforms and eighteenth-century wigs/hats obscuring hair makes me find trying to picture soldier characters a bit of a puzzle!