Is the oil pan still off or have you installed the engine already? The only way to find out is look at the underside of the piston for a part number . Maybe a Ford guy can recognize them.
Pistons w/the machine tooling locating divot like that aren't seem much any more, but they used to be common. Mainly on pistons like Badger and Silvolite. Good chance they're cast non hypereutectic.http://i173.photobucket.com/albums/w66/xzero117/IMG003.jpg
these are stamped .60 over
anyone know what brand they might be or if they're cast or Hypereutectic