So the off side, then? Mine has elastic bands to attach...somehow...I'm just not quite sure how it's supposed to work. And if there's an etiquette on the correct side to put on.
if you put the number so the back is on the throatlash (bridle on horse) pull the bit of elastic around the throatlash (either once or twice depending on how big elastic is) so it is looped and tight around the throatlash and then then end of the elastic loop should tuck over the hook - then do the same thing for the bottom one.
Don't think there is a convention as such as long as it is clearly visible. ie if you have a native with long flowing mane on off side then put it on near side.....
depending on how it attaches the ones I've used (rigid circle ones from BD) fit on off side better as no buckles