Many pistons have a mark, notch, arrow, etc. to show "front" of the piston.
In your case, the photo shows how the pistons are installed:
EDIT- The photo above shows a basically stock piston, if you look closely you can see the front orientation on the crown, pointing to the left. The last photo (I removed it and replaced w/this one) I'm unsure of the way the slot was slanting- but it should have been correct anyway.
Unless there are "left bank" and "right bank" pistons, one bank will have the slot on the major thrust side, the other bank will have the slot on the minor thrust side.
So unless there is a pin offset
or a relief in the piston crown
facing the valves, it really doesn't matter how they're installed, IMO.