Odd and even 4D56 pistons different?

I was pleasantly surprised to find some oversized 4D56 pistons on Amayama, but was confused when I noticed that some were notated "1&3" and others "2&4". Then I saw some aftermarket ones on eBay (Tornaumotoren in Germany) that had 1/3 and 2/4 stamped on top. WHAT'S THE DEAL? Are they really different, odd-from-even?
I built a 4D55 some years back, and the ITM slugs I used were all identical (came as a boxed set). Additionally, I've seen 4-piston aftermarket sets for 4D56 with no such markings (all the same). Can somebody straighten me out on this?