[Mart]

There are heads that just have an A on them. They are compatible with the earlier motors with the valves closer to the cylinders. If I remember correctly they have a relatively small combustion chamber and can offer an increase in compression ratio. They might give valve clearance problems if used on a 59 type motor unless the valve picket area is relieved.
Try looking along the top edge just above the deck face. Sometimes there is a part number cast in along there.
Mart.