I did a thread on yamahaenduro.com about the pistons on the 76 YZ 400. One of the guru's on there gave me the info on the two different rods in the motors. My motor has a 214 K rod in it which is the long rod. The superseded part number for the piston is 2k8-11636-00. Another thing I was told is that the piston for this rod should have 51004 or 51005 inside the skirt.
If the piston you have has 51000, 51002, 51003 inside the skirt, it is the other rod. They didn't give me the number on that rod unfortunately.
I was told the rod difference is 5mm and that is why they moved the piston pin location.
I hope this helps cause it had me confused also. I thought they would all be the same but I saw two different part numbers so that is why I asked.