Hi Gitto and Smithsax. My apologies, Gitto, maybe I was confusing you for S_Pugh, for having hare nightmares. Or maybe you both did...
I colour-coded the extra letters, so I'm pretty sure that I have this right. There are 18 extra letters; of which 5 appear in the perimeter. You're correct that the rest all appear in targets, in fact 3 of the perimeter ones also do.
You're both right about the hare. It is used but not in a mammalian form. The ten targets are indeed of a very specific kind.