big.d.y

Hi everyone. I'm new to the forum. In fact this is my first post in any forum. My son recently purchased a 1973 Yamaha 125. We need parts for it and I am having trouble figuring out if it is an AT1 or AT3. The model number is AT1-228121 so I assumed AT1 but I have been told by motorcycle parts places that Yamaha didn't make a 125 AT1 in 73 but they did make an AT3. Of course they couldn't tell me how to tell what series I had. Could somebody please tell me how I can tell. I also have a 1978 Yamaha 250 that I thought was a DT but again, I am being told there was no 78 DT 250's. How can I tell what series is this machine also? Thanks to all who check this out.