Flat rivets can be used to attach the back strap to the medial side (inside) of the boot permanently. They are especially useful if your horse has a narrow gait and tends to brush feet to each other, which can cause the back straps to come undone when the standard ball rivets are used.
Attaching the back strap with the flat rivets instead, and cutting off any extra strap beyond the rivet closure will ensure the strap stays closed. But keep in mind that it’s not quick to open the flat rivets once they are attached (you need a screwdriver to undo them), so make sure you get the fit right when putting the flat rivets on.
Flat rivets are not included in the standard Flex Boot set when you purchase a boot, but they can be purchased separately.
The rivets are one-size-fits-all, so the same flat rivets will fit all Flex Boot sizes.