to me it looks like a BB 727.....The SB one has got the starter in a little lower location. AFAIK at BB Trans the starter is about same height as the input shaft, SB Trans starter is below input shaft, S6 starter is higher than input shaft.
I was hoping someone could decipher the casting numbers on the side. But, I agree, it does look like the starter area is about in the middle. I guess I'll have to get the stamped numbers on the front rail driver's side??