English to Italian Other Furniture / Household Appliances mounts

non capisco a quale aperture size si riferisca dato che non vengono citate macchine fotografiche

https://www.amazon.co.uk/dp/B00PUZD7WS?th=1

You are purchasing A SET OF 10 Picture mounts.
The first size stated is the outer size of the mount and the second size is the size of the image that will fit inside the mount.
Should the aperture size not be quite right for your images we can make them to your own specifications at no extra cost.
(All 10 mounts must be the same aperture.)