A 5 1/4 inch disk or the floppy variety that has been modified to allow both sides of the disks to be used in a Single Sided 5 1/4 inch disk drive by Flipping the disk over to allow that other side to be read or written to.

The modification involves cutting a write protect notch on the other side of the disk. (This trick was sufficient for some non standard interfaces) and adding a holes in the cover of the disk so that the index hole could be read when the disk is flipped. (See the 5 1/4 inch disk writeup)