Is this used when installing Isofix?

Hi Kyla, Thank you for your enquiry, The Diono Locking Clip is to stop the seat belt moving, The Lap Strap that you are missing can be purchased, we would advise calling your local Baby On The Move store and talking to one of our Trained  Car Restraint Technicians to see if they can install the seat first, there maybe a small charge but safer in the end. I hope this helps. If you follow the link https://www.babyonthemove.co.nz/store-locations/ this will take you to our online store finder where you can find contact details for your nearest Baby On The Move Store.