One is on the cylinder head in between the first and second vane...
One cast onto the side of the lifter area (small)
then theres a great big one INSIDE the lifter area....
13 is a 5 horse, 10 is a 4 horse... Being black it could be either or.. But from looking at the head, I would say it's a 5 horse, or at least a 5 horse head.. Being the 4 horse heads had a tighter straiter grained pattern cooling fins around the spark plug area...