Did the disks for the original IBM 23FD "Minnow" floppy disk also have the sector/index hole near the edge, like the Memorex 650/651? The Minnow was used as a read-only device for loading microcode, and the disks were only factory written. I haven't seen an actual Minnow disk, but since Memorex was one of the companies making plug-compatible equipment, I wouldn't be surprised if they designed the 650/651 medium to be mechanically interchangeable with the 23FD disks.
The Memorex 650/651 supported read/write use, and had more than twice the formatted capacity of the 23FD, so the on-disk format clearly didn't match the 23FD.