A while ago I learned that very early versions of 3.5" floppies didn't have the auto shutter mechanism and you had to open and close the shutter manually. (I think that was probably just used on some Japanese Sony machines.)
Interesting! I tried to find a demonstration of the manual shutter. This is the closest thing I’ve been able to find. Apparently, it was one of the earliest Sony models to feature a semi-automatic shutter.