BB 915 heads look like a 906 from the outside, they have the same ports as a 906, and the same combustion chambers as a 516.
The bb 915 head has the same intake port as the 906 head, but the exhaust port is unique to the 915. It looks very similar, but the short turn/floor is a little lower on the 915. It's a subtle difference.
I haven't had a 915 and 516 next to each other in a very long time, but the way I remember it is there are some subtle changes in the chambers between the two(but I wouldn't swear to that).
