Confused

Hello first time on this forum. I just bought a 1970 Yamaha 125 enduro. Yellow tank. The engine case and frame calls it an AT1. It’s got 5600 miles on it. The stock piston is toasted. I’m having a really hard time finding the right 2nd over piston. I’ve received 2 both are wrong. To big by a lot. I’m obviously doing something wrong. I’ve bought a 285-11636-00-00 and 251-11636-00-00 both are like way ttoobig. Stock bore is 56mm witch is like 2 1/4 inch diameter just for an example. The ones I’ve received are like 2 5/8 inch diameter. I’m getting a mm gauge tomorrow. Point is what am I doing wrong? Any help would be very much appreciated.