I am perplexed by your use of the term concave since that implies that it is dished in.
If not a BB, she is at least part BB based on how heavy her legs are.
Shape of the breast. A heritage turkey will have a V shaped breast while a BB will have a flat, wide breast. Size of the legs and feet is another feature difference. Heritage have slim, long legs with slim toes. BB have short stocky legs with heavy toes.