A spare shirt collar.
This is an unused, vintage item, probably made in the 1960s or thereabouts, when it was usual for shirts to be sold with a replacement collar.
Note: this is a "soft" collar - the sort which you sew on to the shirt, not the sort which fastens on to your shirt with studs.
Suitable to use on a plain shirt (unless of course you happen to have a shirt in the exact same fabric as this collar, lurking in the back of your wardrobe!).
There are removable collar stays (also called stiffeners, or bones) in the collar tips.
Cotton with a crisp white striped pattern.
Available in a choice of colours: