I believe some early 125s had a separate, bolt on drum but the majority had a conventional one piece drum/hub.
See the screen shot from the 200 manual below showing the separate drum.
Although the view of the hub is not clear you can see that the usual 125 one piece hub is a much simpler affair.
Bottom line is, it depends which type you have.
I don't know for sure, but I wouldn't be surprised if you could fit either wheel to your bike.