That was an era when many manufacturers, even high end ones, were switching to plastic gears. Often the internal gears were the first to change. That means if you look at the dears you can see they will be metal. But some of the most critical gears inside were made of the early palstic materials. I don't know about the specifics of this model, but many were like this. Listen to the machine very closely. Any unevenness or clashing sound is a bad signal and needs to be checked out as soon as possible as it could be a signal of internal gear failure.
Bernina has an excellent brand reputation, but this was a transition period in the machines. You might want to have it checked to make sure it is solid.