Old Hardware on Sliding Closet Door
I'm not sure, either. Looking at your first photo, though, it looks like your top mirror guide is missing wheels. There should be wheels that go on those little nubs, which then run along the track as you slide the door open and close.
I won't lie to you, just eyeballing these parts I can tell that they're very old. You may not have a lot of luck finding replacement hardware for them. All I can tell you from this vantage is that it looks like your top guide is missing rollers. That may account for why you can't put them back in the track. It would make sense; if there are only those nubs, they'd fall right out. But that's an educated guess.
You might want a second opinion on this. A local professional who is familiar with homes in the area may have run across this door before and can offer better insight than myself.
That may indeed be what's necessary. I'm sorry again that I couldn't offer any deeper insight. Best of luck to you!