Twelve South Support Product Support BackPack for Mac Where are the numbers on the inserts? Created: August 26, 2018 07:15 The Fit Insert numbers are located inside the insert at the top of the arch. Wondering which inserts to use? Click here to view the BackPack insert chart. To return to the BackPack product page, please click here. Related articles How soon can I track my order? BackPack for iMac Owner's Manual Can I use multiple BackPacks at once?