5.25" disks also had a write protect tab. While the 3.5" disks had a slightly better engineered embedded plastic tab to move up and down the 5.25" disks generally just had a metal clip that served the same purpose. When this wasn't there (which was pretty the case for purchased software disks) there was just a tab cut out instead and to write-enable a disk one just had to put a label (or other tape) over the space; Many earlier used a mechanical device for detecting this slot, later units tended towards optical sensor therefore as long as it was solid and opaque it worked.
This image: https://upload.wikimedia.org/wikipedia/commons/2/24/Dysan_Verbatim_5.25_disks_1978.jpg shows both a metal clip and metal over the space.
Here's one with the write protect slot cut out: http://www.fileformat.info/media/5.25-floppy/top.jpg
(images provided through nothing more than just a google image search).
Biting the hand that feeds IT
