Jim, there is a "void" on about 30/40% of the circumference of the pistion. That void is supposed to be oriented down and toward the rear (rotated) so that it is on a 30 degree angle to the base of the caliper housing.
I have no idea if that makes any sense as it is written and without a picture that is worth a thousand words so I will just keep writing until I get to one thousand words in this post.
If no one else comes up with a link or a picture, if you want to call me I can tell you what I am trying to say. If necessary, I can text you a picture of my $847 Porsche tool that orients the piston correctly. You can make one for $.17 worth of al.
Big Dog