I had to buy a starter for my Sanglas 400Y with Yamaha XS400 engine. When I got it I tore it apart to make sure everything was good before installation. It seemed very similar to the one I already torn apart from my CB350, no wonder as they both are built by mitsuba. Long story short, the rotor, planetary etc were the same, only the mounting points were different.
Actually, I decided it would be a good idea to put new brushes as they were half worn. As I didn't want to spend a week waiting for Yamaha parts, I just dismantled the CB350 starter that was not yet installed, and installed into the XS400 starter. Even the o-ring seal was exactly the same, as well as the sprocket.