High density 1.2m 5.25" diskettes do not have the reinforcing ring, 360k do.
The notcher was to use single sided disks on both sides. There was also a punch to make a 720k disk 1.44m.
You could usually get away with the 720>1.44m trick but you couldn't use 360k disks at 1.2 or vice versa. 360k disks are ferric oxide and 1.2m are Cr02.
If you had an IBM PS/2 you could use a 3.5" disk at any density you wanted because they didn't have the sense pin to see the density hole. It was all based on what format you chose to write them in although you did have to bulk erase them when you changed density. (crap in the gap problem)
There was also a 2.4m 5.25" and a 2.88m 3.5" diskette but they were pretty rare.
