Machined boss on piston - any purpose ?

[Bmacleod]

My query was primarily related to what its purpose may be

Here is a pic of some piston castings. The machined spot in the center of yours is simply where they removed the excess from where they poured the molten metal into the casting. They removed what they thought was all that was necessary to remove and put a small dimple in the middle to locate the center for further machining.