Sent this one off to Mike Diamond for an in hand look. For those curious, here is his analysis:
Your Buffalo nickel
arrived yesterday. I took a look at it and cannot arrive at a definitive conclusion regarding the authenticity of the defect. However, I lean toward post-strike damage, for three reasons. First, I see no metal flow in the bison's tail where it borders the sloping surface. Second, radial flow lines show no change as they approach the sloping surface. These would be affected by metal flow as well. Third, the concave field terminates too abruptly when it meets the sloping surface at 3:00 on the reverse face. If this defect was present before the strike, the junction between field and defect would be less sharply defined.