Yamadt100

So if they r interchangeable, why do they have different numbers? what is te difference between them?

Okay guys, so I have a dt-100a motor (437-107460) and the parts interchangeable with a dt100g (3a3-072702)..... I have a dt100 motor in my bike now and the bottom end is shot but the piston rings and jug are new. I also have a dt100g with a good bottom end I want to put the cylinder piston clutch and other odds and ends on the dt100g bottom end..... Will it work? Thanks for the responds. And sorry for my grammer in a hurry lol