This type of uke system is adjustable so it can be used with most uke sizes and is designed so that no strap buttons are needed.
U-66-BLK Uke Strap in Black Leather & Striped Nylon
• Soft top grain black leather & suede shoulder pad
• Malleable double hooks covered with top grain leather
• High quality adjustable nylon webbing
• Fits most ukes & many mandolins
• Adjustable from 39”-56″
Ukes almost never come with strap buttons or any other good way to attach a strap. The typical neck strap with a single hook works well enough, except that if you ever let go of the uke it will fall down. This new double hook system is the solution to that problem. It allows you to wear the strap over your shoulder like you normally would with a guitar strap, instead of around your neck, and the double hooks mean you can let go of the uke without it falling to the floor.
Here’s a link to the instructions for attaching the U-66 to your ukulele.