A spare shirt collar.
This is an unused, vintage item, probably made in the 1970s 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.
I am not quite sure what the fabric is - possibly a polyester cotton, or trubenised cotton?