Got it figured out
I took the piston to my nearest KTM dealer. After humbling myself by saying I would not buy KTM parts by mail order any more, the mechanic straightened me out.
First you are right, Elf, the #2 piston is slightly larger, and for some reason mine came with one from the factory.
Secondly, we looked a a bunch of KTM pistons, none of them had their rings line up like these did. The mechanic looked a bit closer, and noted that the ring end gap was lined up in a transfer port. If I had installed it, the ring end would have popped into the port and maybe broke a ring, or even worse, ruined the bore.
The new OEM piston was defective! He couldn't believe KTM's piston supplier screwed up like that. He had never seen it before. I returned it to the mail order supplier for credit.
To follow up, the mechanic miked my old piston and my bore, and told me I was wasting money changing my old piston, It was absolutely fine. KTM pistons are high quality units that normally last for years under normal use. It is not like a Jap bike where you do need to replace the piston just because. He sold me a set of rings, the bike is back together, and runs just fine.
The moral of the story for me is pay a few extra bucks and take care of my local dealer.